
He announces this with his opening words, and will then continue to, well basically, just about beat us over the head with it, over and over and over and over again. By the end of this chapter, we will have heard the word "day" at least a dozen times, and the phrase "And on that day" - ביום ההוא - seven times.
(Remember the time we said that "seven" is a magic number, and if something is repeated seven times, then it is a sure sign that we're dealing with something important? (See A Kingmaker is Born: https://www.sefaria.org/sheets/263443?lang=bi). Well, the same applies here!)
So what is this big "Day" that is upon us? Let's just start out by looking at Zecharia's opening declaration; (WARNING; graphic and sensitive content ahead here.)

(א) הִנֵּ֥ה יֽוֹם־בָּ֖א לַֽיהוָ֑ה וְחֻלַּ֥ק שְׁלָלֵ֖ךְ בְּקִרְבֵּֽךְ׃ (ב) וְאָסַפְתִּ֨י אֶת־כָּל־הַגּוֹיִ֥ם ׀ אֶֽל־יְרוּשָׁלִַם֮ לַמִּלְחָמָה֒ וְנִלְכְּדָ֣ה הָעִ֗יר וְנָשַׁ֙סּוּ֙ הַבָּ֣תִּ֔ים וְהַנָּשִׁ֖ים תשגלנה [תִּשָּׁכַ֑בְנָה] וְיָצָ֞א חֲצִ֤י הָעִיר֙ בַּגּוֹלָ֔ה וְיֶ֣תֶר הָעָ֔ם לֹ֥א יִכָּרֵ֖ת מִן־הָעִֽיר׃ (ג) וְיָצָ֣א יְהוָ֔ה וְנִלְחַ֖ם בַּגּוֹיִ֣ם הָהֵ֑ם כְּי֥וֹם הִֽלָּחֲמ֖ו בְּי֥וֹם קְרָֽב׃
(1) Lo, a day of YHWH is coming, when your spoil shall be divided in your very midst! (2) For I will gather all the nations to Jerusalem for war!
The city shall be captured, the houses plundered, and the women violated.

But as bad as things appear... sorry to say that they are even worse if we understand a bit of context.
You might recall when we spoke at length about Zecharia in the Branch of David limmud (https://www.sefaria.org/sheets/244292?lang=bi). His time-and-place was Jerusalem after the return from exile, when the Judeans are in a state of rebuilding! But what Zechariah is describing here is the worst of nightmares; something they had already suffered through at the hands of Babylon, 70 years ago!
Is this to say that the Judeans, after enduring a miserable destruction-and-exile, are only redeemed and brought back to Judea only to experience the same thing all over again???
What ever happened to all the assurances from the Herald of Zion of a renewed and reconciled relationship, and living happily-ever-after!

And in this case, we only need to go one line further to where the pivot begins:
(ג) וְיָצָ֣א יְהוָ֔ה וְנִלְחַ֖ם בַּגּוֹיִ֣ם הָהֵ֑ם כְּי֥וֹם הִֽלָּחֲמ֖ו בְּי֥וֹם קְרָֽב׃
(3) Then YHWH shall sally forth to wage war on those nations as he is wont to make war on a day of battle.
And it's not enough for YHWH to show up and fight; No, he has to make an appearance, showing up in such dramatic fashion so as the whole of the cosmos will stop and look; Such that everyone will see what happens when Judea, YHWH's precious people, are trampled upon.
Look at how Zecharia goes on to describe this moment:
(ד) וְעָמְד֣וּ רַגְלָ֣יו בַּיּוֹם־הַ֠הוּא עַל־הַ֨ר הַזֵּתִ֜ים אֲשֶׁ֨ר עַל־פְּנֵ֥י יְרוּשָׁלִַם֮ מִקֶּדֶם֒ וְנִבְקַע֩ הַ֨ר הַזֵּיתִ֤ים מֵֽחֶצְיוֹ֙ מִזְרָ֣חָה וָיָ֔מָּה גֵּ֖יא גְּדוֹלָ֣ה מְאֹ֑ד וּמָ֨שׁ חֲצִ֥י הָהָ֛ר צָפ֖וֹנָה וְחֶצְיוֹ־נֶֽגְבָּה׃ (ה) וְנַסְתֶּ֣ם גֵּֽיא־הָרַ֗י כִּֽי־יַגִּ֣יעַ גֵּי־הָרִים֮ אֶל־אָצַל֒ וְנַסְתֶּ֗ם כַּאֲשֶׁ֤ר נַסְתֶּם֙ מִפְּנֵ֣י הָרַ֔עַשׁ בִּימֵ֖י עֻזִּיָּ֣ה מֶֽלֶךְ־יְהוּדָ֑ה וּבָא֙ יְהוָ֣ה אֱלֹהַ֔י כָּל־קְדֹשִׁ֖ים עִמָּֽךְ׃ (ו) וְהָיָ֖ה בַּיּ֣וֹם הַה֑וּא לֹֽא־יִהְיֶ֣ה א֔וֹר יְקָר֖וֹת יקפאון [וְקִפָּאֽוֹן׃] (ז) וְהָיָ֣ה יוֹם־אֶחָ֗ד ה֛וּא יִוָּדַ֥ע לַֽיהוָ֖ה לֹא־י֣וֹם וְלֹא־לָ֑יְלָה וְהָיָ֥ה לְעֵֽת־עֶ֖רֶב יִֽהְיֶה־אֽוֹר׃
(4) On that day, he will set his feet on the Mount of Olives, near Jerusalem on the east; and the Mount of Olives shall split across from east to west, and one part of the Mount shall shift to the north and the other to the south, a huge gorge.
(5) And the Valley in the Hills shall be stopped up, for the Valley of the Hills shall reach only to Azal; it shall be stopped up as it was stopped up as a result of the earthquake in the days of King Uzziah of Judah.—And YHWH my Elohim, with all the holy beings, will come to you.
(6) In that day, there shall be neither sunlight nor cold moonlight, (7) but there shall be a continuous day—only YHWH knows when—of neither day nor night, and there shall be light at eventide.

What Zecharia is describing here is called theophany, and it refers to when YHWH decides to show up somewhere amongst humankind, and the ancient Judeans and Israelites usually described such appearances as being accompanied by such awe-evoking events as booming thunder and earthquakes.
Here is one of my personal favorite examples of this:
(ז) בַּצַּר־לִ֤י ׀ אֶֽקְרָ֣א יְהוָה֮ וְאֶל־אֱלֹהַ֪י אֲשַׁ֫וֵּ֥עַ יִשְׁמַ֣ע מֵהֵיכָל֣וֹ קוֹלִ֑י וְ֝שַׁוְעָתִ֗י לְפָנָ֤יו ׀ תָּב֬וֹא בְאָזְנָֽיו׃ (ח) וַתִּגְעַ֬שׁ וַתִּרְעַ֨שׁ ׀ הָאָ֗רֶץ וּמוֹסְדֵ֣י הָרִ֣ים יִרְגָּ֑זוּ וַ֝יִּתְגָּֽעֲשׁ֗וּ כִּי־חָ֥רָה לֽוֹ׃ (ט) עָ֘לָ֤ה עָשָׁ֨ן ׀ בְּאַפּ֗וֹ וְאֵשׁ־מִפִּ֥יו תֹּאכֵ֑ל גֶּ֝חָלִ֗ים בָּעֲר֥וּ מִמֶּֽנּוּ׃ (י) וַיֵּ֣ט שָׁ֭מַיִם וַיֵּרַ֑ד וַ֝עֲרָפֶ֗ל תַּ֣חַת רַגְלָֽיו׃ (יא) וַיִּרְכַּ֣ב עַל־כְּ֭רוּב וַיָּעֹ֑ף וַ֝יֵּ֗דֶא עַל־כַּנְפֵי־רֽוּחַ׃ (יב) יָ֤שֶׁת חֹ֨שֶׁךְ ׀ סִתְר֗וֹ סְבִֽיבוֹתָ֥יו סֻכָּת֑וֹ חֶשְׁכַת־מַ֝֗יִם עָבֵ֥י שְׁחָקִֽים׃ (יג) מִנֹּ֗גַהּ נֶ֫גְדּ֥וֹ עָבָ֥יו עָבְר֑וּ בָּ֝רָ֗ד וְגַֽחֲלֵי־אֵֽשׁ׃ (יד) וַיַּרְעֵ֬ם בַּשָּׁמַ֨יִם ׀ יְֽהוָ֗ה וְ֭עֶלְיוֹן יִתֵּ֣ן קֹל֑וֹ בָּ֝רָ֗ד וְגַֽחֲלֵי־אֵֽשׁ׃ (טו) וַיִּשְׁלַ֣ח חִ֭צָּיו וַיְפִיצֵ֑ם וּבְרָקִ֥ים רָ֝ב וַיְהֻמֵּֽם׃ (טז) וַיֵּ֤רָא֨וּ ׀ אֲפִ֥יקֵי מַ֗יִם וַֽיִּגָּלוּ֮ מוֹסְד֪וֹת תֵּ֫בֵ֥ל מִגַּעֲרָ֣תְךָ֣ יְהוָ֑ה מִ֝נִּשְׁמַ֗ת ר֣וּחַ אַפֶּֽךָ׃
(7) In my distress I called on YHWH, cried out to my elohim; in his temple he heard my voice; my cry to him reached his ears!
(8) Then the earth rocked and quaked; the foundations of the mountains shook, rocked by his indignation; (9) smoke went up from his nostrils, from his mouth came devouring fire; live coals blazed forth from him.
(10) He tilted the sky and came down, thick cloud beneath his feet. (11) He mounted a cherub and flew, gliding on the wings of the wind. (12) He made darkness His screen; dark thunderheads, dense clouds of the sky were his pavilion round about him. (13) Out of the brilliance before him, hail and fiery coals pierced his clouds.
(14) Then YHWH thundered from heaven, the Most High gave forth his voice— hail and fiery coals. (15) He let fly His shafts and scattered them; He discharged lightning and routed them. (16) The ocean bed was exposed; the foundations of the world were laid bare by Your mighty roaring, O YHWH, at the blast of the breath of Your nostrils!

For the Israelites and Judeans, their conviction in YHWH as a mighty warrior-figure who would fight for them, defend them, sally forth before them in battle, was one of the most important roles YHWH could fill. After all, the Israelites and Judeans were surrounded by hostile kingdoms and empires on all sides, and were constantly having to go to war to defend their cities, their homes and families. In the ancient near east, a god who was not capable of or willing to successfully wage war for his people was not worthy of worship.
And in this arena, our Israelite and Judean ancestors believed that YHWH proved himself time and time again.
The most concise statement throughout the writings in the TaNaKh which expresses this confidence comes from the Song of the Sea, sung daily in traditional Jewish prayer-spaces:

There's much to say about the earthquake from the days of King Uzziah. Take a look at how Amos, one of the four great 8th-century prophets, and most likely the oldest prophet to have his own oracles in our TaNaKh, opens up his works:
(א) דִּבְרֵ֣י עָמ֔וֹס אֲשֶׁר־הָיָ֥ה בַנֹּקְדִ֖ים מִתְּק֑וֹעַ אֲשֶׁר֩ חָזָ֨ה עַל־יִשְׂרָאֵ֜ל בִּימֵ֣י ׀ עֻזִיָּ֣ה מֶֽלֶךְ־יְהוּדָ֗ה וּבִימֵ֞י יָרָבְעָ֤ם בֶּן־יוֹאָשׁ֙ מֶ֣לֶךְ יִשְׂרָאֵ֔ל שְׁנָתַ֖יִם לִפְנֵ֥י הָרָֽעַשׁ׃
(1) The words of Amos, a sheepbreeder from Tekoa, who prophesied concerning Israel in the reigns of Kings Uzziah of Judah and Jeroboam son of Joash of Israel, two years before the earthquake.
This earthquake was so memorable that all he had to say was "the earthquake," and he could assume that as long as Amos's words would be read, everyone in the audience would know exactly what he was talking about.
Here is a description from The Scientific and Scriptural Impact of Amos' Earthquake by Steven Austin, PhD:
...
After the gigantic earthquake, no Hebrew prophet could predict a divine visitation in judgment without alluding to an earthquake.
from The Scientific and Scriptural Impact of Amos' Earthquake, Steven Austin, PhD., 2010 : https://www.icr.org/article/scientific-scriptural-impact-amos-earthquake/

https://www.sefaria.org/sheets/262041.9?lang=bi&with=Sheets&lang2=en
Firstly; note how one of YHWH's most popular epithets throughout the literature in the TaNaKh is YHWH-of-Legions, or יהוה צבאות . The first time we encounter this word - legions - is actually as early as the conclusion of the first creation narrative in Genesis:
(לא) וַיַּ֤רְא אֱלֹהִים֙ אֶת־כָּל־אֲשֶׁ֣ר עָשָׂ֔ה וְהִנֵּה־ט֖וֹב מְאֹ֑ד וַֽיְהִי־עֶ֥רֶב וַֽיְהִי־בֹ֖קֶר י֥וֹם הַשִּׁשִּֽׁי׃ (פ) (א) וַיְכֻלּ֛וּ הַשָּׁמַ֥יִם וְהָאָ֖רֶץ וְכָל־צְבָאָֽם׃
(31) And Elohim saw all that he had made, and found it very good. And there was evening and there was morning, the sixth day. (1) And the heaven and the earth were finished, and all their legions.
In fact, look at how Deborah, the incredible pre-monarchic heroine-prophetess, poetically depicts one of the ancient Isrealite-Canaanite wars:
(יט) בָּ֤אוּ מְלָכִים֙ נִלְחָ֔מוּ אָ֤ז נִלְחֲמוּ֙ מַלְכֵ֣י כְנַ֔עַן בְּתַעְנַ֖ךְ עַל־מֵ֣י מְגִדּ֑וֹ בֶּ֥צַע כֶּ֖סֶף לֹ֥א לָקָֽחוּ׃ (כ) מִן־שָׁמַ֖יִם נִלְחָ֑מוּ הַכּֽוֹכָבִים֙ מִמְּסִלּוֹתָ֔ם נִלְחֲמ֖וּ עִם־סִיסְרָֽא׃ (כא) נַ֤חַל קִישׁוֹן֙ גְּרָפָ֔ם נַ֥חַל קְדוּמִ֖ים נַ֣חַל קִישׁ֑וֹן תִּדְרְכִ֥י נַפְשִׁ֖י עֹֽז׃ (כב) אָ֥ז הָלְמ֖וּ עִקְּבֵי־ס֑וּס מִֽדַּהֲר֖וֹת דַּהֲר֥וֹת אַבִּירָֽיו׃ (כג) א֣וֹרוּ מֵר֗וֹז אָמַר֙ מַלְאַ֣ךְ יְהוָ֔ה אֹ֥רוּ אָר֖וֹר יֹשְׁבֶ֑יהָ כִּ֤י לֹֽא־בָ֙אוּ֙ לְעֶזְרַ֣ת יְהוָ֔ה לְעֶזְרַ֥ת יְהוָ֖ה בַּגִּבּוֹרִֽים׃
(19) Then the kings came, they fought: The kings of Canaan fought At Taanach, by Megiddo’s waters— They got no spoil of silver. (20) The stars fought from heaven, From their courses they fought against Sisera. (21) The torrent Kishon swept them away, The raging torrent, the torrent Kishon.
March on, my soul, with courage!
(22) Then the horses’ hoofs pounded as headlong galloped the steeds.
(23) “Curse Meroz!” said the agent of YHWH.
“Bitterly curse its inhabitants, Because they came not to the aid of YHWH, to the aid of YHWH among the warriors!”
An ancient Israelite poet described the vastness of YHWH's forces thusly:
(יח) רֶ֤כֶב אֱלֹהִ֗ים רִבֹּתַ֣יִם אַלְפֵ֣י שִׁנְאָ֑ן אֲדֹנָ֥י בָ֝֗ם סִינַ֥י בַּקֹּֽדֶשׁ׃
(18) Elohim's chariots are twice ten thousand, with thousands doubled! Adonai is among them.

We'll come back to this plague in a moment. But first; if you're at least 25 years old, chances are you have been privileged enough to see the original Indiana Jones classic, Raiders of the Lost Ark. And if so, if there's one scene that you would remember forever, it was probably the moment when the Ark of the Covenant is found and opened up.
Here it is below; but WARNING, it is a scene of nightmares, not for the faint of heart.
(יב) וְזֹ֣את ׀ תִּֽהְיֶ֣ה הַמַּגֵּפָ֗ה אֲשֶׁ֨ר יִגֹּ֤ף יְהוָה֙ אֶת־כָּל־הָ֣עַמִּ֔ים אֲשֶׁ֥ר צָבְא֖וּ עַל־יְרוּשָׁלִָ֑ם הָמֵ֣ק ׀ בְּשָׂר֗וֹ וְהוּא֙ עֹמֵ֣ד עַל־רַגְלָ֔יו וְעֵינָיו֙ תִּמַּ֣קְנָה בְחֹֽרֵיהֶ֔ן וּלְשׁוֹנ֖וֹ תִּמַּ֥ק בְּפִיהֶֽם׃
(12) And this shall be the plague that YHWH will plague all of the nations who arrayed against Jerusalem;
Their flesh shall rot away while they stand on their feet; Their eyes shall rot in their sockets; Their tongues shall rot in their mouths.
One major highlight in Zechariah's oracle which we can't skip over is what ends up happening with all of those enemies who, for some reason, end up surviving this major day of reckoning:
(טז) וְהָיָ֗ה כָּל־הַנּוֹתָר֙ מִכָּל־הַגּוֹיִ֔ם הַבָּאִ֖ים עַל־יְרֽוּשָׁלִָ֑ם וְעָל֞וּ מִדֵּ֧י שָׁנָ֣ה בְשָׁנָ֗ה לְהִֽשְׁתַּחֲוֺת֙ לְמֶ֙לֶךְ֙ יְהוָ֣ה צְבָא֔וֹת וְלָחֹ֖ג אֶת־חַ֥ג הַסֻּכּֽוֹת׃
(16) All who survive of all those nations that came up against Jerusalem shall make a pilgrimage year by year to bow low to the King, YHWH-of-Legions, and to observe the Feast of Booths.

(מב) בַּסֻּכֹּ֥ת תֵּשְׁב֖וּ שִׁבְעַ֣ת יָמִ֑ים כָּל־הָֽאֶזְרָח֙ בְּיִשְׂרָאֵ֔ל יֵשְׁב֖וּ בַּסֻּכֹּֽת׃ (מג) לְמַעַן֮ יֵדְע֣וּ דֹרֹֽתֵיכֶם֒ כִּ֣י בַסֻּכּ֗וֹת הוֹשַׁ֙בְתִּי֙ אֶת־בְּנֵ֣י יִשְׂרָאֵ֔ל בְּהוֹצִיאִ֥י אוֹתָ֖ם מֵאֶ֣רֶץ מִצְרָ֑יִם אֲנִ֖י יְהוָ֥ה אֱלֹהֵיכֶֽם׃ (מד) וַיְדַבֵּ֣ר מֹשֶׁ֔ה אֶת־מֹעֲדֵ֖י יְהוָ֑ה אֶל־בְּנֵ֖י יִשְׂרָאֵֽל׃ (פ)
(42) You shall live in booths seven days; all citizens in Israel shall live in booths, (43) in order that future generations may know that I made the Israelite people live in booths when I brought them out of the land of Egypt, I YHWH your elohim!
Note how the above command even emphasizes that this is a law for ezrach
b'Israel, the "citizens" of Israel.
So the question is begged; Why would YHWH now broaden the Festival of Booths to include all of those nations who tried to make war? YHWH didn't free them from Egypt and accompany them for 40 years; It was Israel! What relevance do they have to this particular pilgrimage festival???
Personally, I think this hearkens back to another oracle of Zechariah's, one several chapters back, and which very well might be one of the most beautiful chapters in all of TaNaKh, up there even with some of the very best we read from the Herald of Zion.
I recommend you read this whole chapter, but here we'll only look at one small part:
(כ) כֹּ֥ה אָמַ֖ר יְהוָ֣ה צְבָא֑וֹת עֹ֚ד אֲשֶׁ֣ר יָבֹ֣אוּ עַמִּ֔ים וְיֹשְׁבֵ֖י עָרִ֥ים רַבּֽוֹת׃ (כא) וְֽהָלְכ֡וּ יֹשְׁבֵי֩ אַחַ֨ת אֶל־אַחַ֜ת לֵאמֹ֗ר נֵלְכָ֤ה הָלוֹךְ֙ לְחַלּוֹת֙ אֶת־פְּנֵ֣י יְהוָ֔ה וּלְבַקֵּ֖שׁ אֶת־יְהוָ֣ה צְבָא֑וֹת אֵלְכָ֖ה גַּם־אָֽנִי׃ (כב) וּבָ֨אוּ עַמִּ֤ים רַבִּים֙ וְגוֹיִ֣ם עֲצוּמִ֔ים לְבַקֵּ֛שׁ אֶת־יְהוָ֥ה צְבָא֖וֹת בִּירוּשָׁלִָ֑ם וּלְחַלּ֖וֹת אֶת־פְּנֵ֥י יְהוָֽה׃ (ס) (כג) כֹּ֥ה אָמַר֮ יְהוָ֣ה צְבָאוֹת֒ בַּיָּמִ֣ים הָהֵ֔מָּה אֲשֶׁ֤ר יַחֲזִ֙יקוּ֙ עֲשָׂרָ֣ה אֲנָשִׁ֔ים מִכֹּ֖ל לְשֹׁנ֣וֹת הַגּוֹיִ֑ם וְֽהֶחֱזִ֡יקוּ בִּכְנַף֩ אִ֨ישׁ יְהוּדִ֜י לֵאמֹ֗ר נֵֽלְכָה֙ עִמָּכֶ֔ם כִּ֥י שָׁמַ֖עְנוּ אֱלֹהִ֥ים עִמָּכֶֽם׃ (ס)
(20) Thus said YHWH-of-Legions:
Peoples and the inhabitants of many cities shall yet come— (21) the inhabitants of one shall go to the other and say,
“Let us go and entreat the favor of YHWH! Let us seek YHWH-of-Legions!; I want to go, me too!”
(22) The many peoples and the multitude of nations shall come to seek YHWH-of-Legions in Jerusalem and to entreat the favor of YHWH.
(23) Thus said YHWH-of-Legions:
In those days, ten men from nations of every tongue will take hold—they will take grasp every Judean by a corner of his cloak and say:
“Let us go with you, for we have heard that Elohim-is-with-you!”
Can you imagine this image? Judeans making their way to Jerusalem during the fall, with their tools to build their booths in one hand, their 4 species in another, provisions on their backs; And as they pass by town after town, people pouring out of their homes begging to be included; And then the city of Jerusalem will become a symphony of brotherhood and unity, with peoples of many colors and cultures and origins, all joining together in fellowship, rendering praise to YHWH!
And as Zechariah foretells:
(ט) וְהָיָ֧ה יְהוָ֛ה לְמֶ֖לֶךְ עַל־כָּל־הָאָ֑רֶץ בַּיּ֣וֹם הַה֗וּא יִהְיֶ֧ה יְהוָ֛ה אֶחָ֖ד וּשְׁמ֥וֹ אֶחָֽד׃
(9) And then YHWH shall be king over all the earth; And on that day YHWH shall be one, and his name one!
Should we, as the descendants of our beloved Israelites and Judeans of yore, be active in spreading the word of YHWH beyond our prayer-spaces and communities to those who do not know of him, or are not in relationship with him? After all, if having YHWH in our lives brings us meaning and fulfillment, is a source of strength and comfort; is it not kindness to try to share this, somehow, with others?
Based on this chapter, my inclination is to say yes; For YHWH's intention, here at least, seems to be for him to be sovereign to the whole world, not just over Israel. If so, how can we play our small part to contributing to YHWH's will?
Here's an idea that you might appreciate starting with; People are oftentimes asked "How are you?" This is a WONDERFUL invitation to call upon the name of YHWH! One can either answer with gratitude for being well, or else a prayer for things to get better.
"I'm great! YHWH-Almighty has been kind to me."
Or alternatively;
"I'm unwell, may YHWH-Almighty help me through this."
Maybe, every once in a while, such a response will open up a conversation of trust and faith in YHWH that will be fruitful and worthwhile for everyone involved.
