1,402. When Purim Falls on Shabbos

Hilchos Megillah v’Chanukah 1:12

If the megillah was read in the first month of Adar but the court subsequently declared a leap year, the megillah must be read again in the second Adar at the appropriate time.

Hilchos Megillah v’Chanukah 1:13

The megillah is not read on Shabbos. This is a preventive measure against the possibility that one might take a megillah to a person who knows how to read it, thereby carrying it in the public domain. Everyone is obligated in the reading of the megillah but not everyone is able to do so for himself, so such an occurrence is likely. This is why, if the appropriate time for reading the megillah falls on Shabbos, we do so earlier, on Friday. We review the laws of Purim on that Shabbos in acknowledgement of the fact that the day is Purim.