Laws of the Megillah
20. A mourner ("avel") during "shivah" (1) should observe all the laws of
mourning [on Purim], and is forbidden to attend any kind of celebration.
However, wearing shoes and sitting on a chair are permitted because these
are activities that are obvious to all (2). It is preferable that he gather
a "minyan" (3) to his home for the Megillah reading, however, if that is
not possible, he should pray at home, and then go to synagogue to hear the
Megillah (4). If [Purim] falls on Saturday night, he should go to synagogue
on Shabbos afternoon after the third meal ("Seuda Shlishis") before sunset
(and stay there for the evening service and Megillah reading) (5). During
the daytime [hours of Purim, the mourner] may go to synagogue for both
praying and to hear the Megillah.
(1) The first seven days after the burial of a member of one's immediate
family (parents, siblings, children or spouse).
(2) There is a dispute among the later authorities ("acharonim") as to
whether the laws of mourning apply on the day that Purim is celebrated. The
present custom is that only the aspects of mourning that can be kept
private ("devorim she'batzinah") are observed (Shulchan Aruch 496:4).
(3) Ten adult males (including the mourner himself).
(4) A mourner during "shivah" is not permitted to leave his house (or the
house where he has chosen to spend the first seven days of mourning),
except for certain circumstances, such as performing certain mitzvos. The
reason for this prohibition is so that the mourner doesn't get distracted
from mourning. Since it is a mitzvah to hear the Megillah with a minyan, if
he can't gather ten men to his home, he may go to synagogue.
(5) A mourner during "shivah" is permitted to go to shul on Shabbos.