ASBI Pocket Beit Midrash: Aruch HaShulchan Orach Chaim 687:2

Our rabbis, the Tosafot, the Rosh, and the Ran, all wrote that the essence of the mitzvah of reading the megilah and publicizing the miracle is during the day and not at night, as it says "I call out each day" that is to say, even though I call out during the day, which is the essential reading, nonetheless, I will not be silent at night. And the essential meal of Purim is also during the day and so too Mishloach Manot. As it says, "and these days shall be remembered and performed." The verse juxtaposes remembering and doing. Just as we do the mitzvot during the day, we remember and recite during the day. Therefore in the Mishnah at the end of the second chapter of Megilah it is taught "the entire day is valid to read megilah." And at night it does not teach that the entire night is valid to read megillah because the essential reading is during the day (Rosh). And for this reason, even though we recite the blessing "She'hechiyanu" on reading the megilah at night we repeat it during the day for the essential reading is during the day. But the Rambam truly rules in the first chapter of Laws of Megilah that we do not repeat the "shehechyanu" blessing during the day and this will be explained in Section 692.