Tosfos’ opinion (Meg 4) is that a bracha of she’hechiyanu is recited before reading megillah during the day even though it has already been recited on the reading of the previous night. The Rambam disagrees, and holds that since she’hechiyanu has been recited at night it is not repeated. I offered a few approaches to understand this machlokes last year, but wanted to add two new ideas this year. The gemara quotes R’ Yehoshua ben Levi as saying one is obligated to read the megillah at night and repeat it during the day, but no mention of this night time reading appears anywhere in the Mishna. Perhaps, some intriguingly suggest, the original takanah of megillah was to read only during the day, with the full set of brachos. The night reading was only a later takkanah made by R’ Yehoshua ben Levi. This idea is consistent with the Turei Even’s chiddush that the reading by day is a chiyuv m’divrei kabbalah, but at night is only a takanah derabbanan. (This idea makes for a difficult reading of a gemara on daf 20 that I don’t want to get bogged down in, so I’ll just leave you with the mareh makom).
Tosfos proves that the day reading is primary from the fact that Purim seudah is done during the day but not the night before. R’ Soloveitchik (quoted in Moadei haRav) used this as a springboard for another interesting distinction between the two readings. The Rav suggested that other Yamim Tovim have a kedushas hayom from which stems all the associated mitzvos of the day. The kedushas hayom of each Yom Tov begins with sunset the previous night. Purim, however, does not have a kedushas hayom which obligates mitzvos. There are mitzvos hayom - mishloach manos, matanos l’evyonim, kerias hamegillah - which must be done on the day of Purim, to the exclusion of the previous night, but nothing more. The reading at night is not part of the mitzvos hayom of Purim, but is just a preparatory act in anticipation of the day of Purim itself.