Most years the Purim se’udah (feast) takes place late on Purim afternoon despite the normal rule that mitzvot should not be delayed. Purim seems an exception to the rule, presumably because the observances of the day include sending gifts of edible foods (mishlo’ach manot) to one’s neighbours and friends, and giving charity to the poor (mattanot la’evyonim). Until these requirements have been met, enabling other people to enjoy the festivities, our own celebrations cannot in all conscience commence.
The Jewish ethos of celebration always says that one must not be selfish or put the needs of others last. This is why every community ensures that the poor have the means to celebrate Pesach and indeed every happy occasion in the year – especially Shabbat. Some of the greatest acts of quiet charity in Israel enable disadvantaged families to have food and wine for Shabbat.
When Purim falls on a Friday, we have to make sure that the feasting does not get in the way of the observance of Shabbat. Since the se’udah can be held at any time during the day, on a Friday it is customary for it to begin earlier than usual. Some communities actually time it for the morning immediately after the M’gillah reading. In cities in Israel (notably Jerusalem) which were surrounded by walls at the time of Joshua, the se’udah is postponed from Friday. Extra delicacies are added to the Shabbat meals in honour of Purim, and the se’udah itself is on Sunday (Shulchan Aruch, Orach Chayyim 688:6).