Megillah

The Megillah is read on the eleventh, the twelfth, the thirteenth, the fourteenth, and the fifteenth [of Adar], never earlier and never later. Cities which have been walled since the days of Joshua ben Nun read on the fifteenth; villages and large towns read on the fourteenth, except that villages move the reading up to the day of gathering.

How so? If the fourteenth [of Adar] falls on Monday, the villages and large towns read on that day and the walled places on the next day. If it falls on Tuesday or on Wednesday, the villages move the reading up to the day of gathering, the large towns read on that day, and the walled places on the next day.

If it falls on Thursday, the villages and large towns read on that day and the walled places on the next day. If it falls on Friday, the villages move the reading up to the day of gathering and the large towns and walled places read on that day.

If it falls on Shabbat, the villages and large towns move the reading up to the day of gathering, and the walled places read on the next day. If it falls on Sunday, the villages move the reading up to the day of gathering, the large towns read on that day, and the walled cities on the day following.