Remembrance precedes the deed

מה שלא קבעו בלילה משתה ושמחה.

Why they did not establish the night for feasting and rejoicing.

The Sefas Emes asks why Chazal fixed the Purim seudah for the day and not for the night.

כי העשי' בא אחר הזכירה.

Because the action comes only after the remembrance.

Doing (asiyah) flows from first recalling the miracle; the deed must be rooted in zechirah (remembrance).

ועיקר הזכירה ביום.

And the essence of the remembrance is by day.

The primary moment of remembering, like all matters of clear recognition, belongs to the daytime.

וכן הוכיחו תוס' מדסעודה ביום דעיקר מצות קריאה ביום.

And so Tosafos proved from the fact that the seudah (festive meal) is by day, that the essential mitzvah of reading the Megillah is by day.

Tosafos derive that since the meal is held during the day, the main obligation of reading the Megillah, which the meal accompanies, is likewise a daytime mitzvah.

ולכך אחר השלמת הזכירה ומתעורר ע"י הקריאה הארת הנס כדכתיב והימים כו' נזכרים ונעשים.

Therefore, after the remembrance is completed and the illumination of the miracle is awakened through the reading, as it is written, "And these days... are remembered and are done" (Esther 9:28).

Only once the zechirah is whole and the reading of the Megillah arouses the light of the nes (miracle) can the "doing" follow — the pasuk itself places "remembered" before "done."

וכן לעשות אותם ימי משתה ושמחה הוא כנ"ל שע"י הקריאה מתעורר הנס:

And so too making them days of feasting and rejoicing follows the same principle stated above, that through the reading the miracle is aroused.

The feasting and simchah of Purim are an outgrowth of the reading, for it is the krias haMegillah that reawakens the miracle each year and makes the celebration possible.

Summary: The Purim seudah is fixed for the day because action (asiyah) must flow from remembrance (zechirah), and the essential remembrance — the reading of the Megillah that reawakens the light of the miracle — belongs to the daytime; "remembered" must precede "done."