Can we offer mishloah manot the night of Purim after the reading of the Megilla? Is it allowed?
The Rema quotes the Rosh as saying to do it during the day. The Mishnah Berurah also says this, as does Rav Ovadia.
The time for the mitzvah is tied to the se'udah which is held during the day.
The Ran (Megilla 3b in the pages of the Rif) addresses the issue. Despite the fact that megilla reading varies, the se’udat Purim does not; it must be scheduled on the 14th or 15th (depending on whether the city is walled). Hence, the Ran claims, since mishloach manot is part of the se’uda it is “tethered” to the se’uda: whenever the se’uda occurs, mishloach manot must be disbursed. ... A third issue relates to the exact time on Purim during which this mitzva should be performed. The Rema (OC 695) cites the position that mishloach manot must be delivered during the day. Many attempt to provide an explanation to support this position (see, for example, the Bi’ur Ha-Gra). One solution might be to invalidate evening mishloach manot because the evening is invalid for the se’uda (see Megilla 7b). Again, affixing mishloach manot to the se’uda would mandate that the manot be sent only during a period which is suitable for se’uda.
Discussion from here.