The following is a brief overview of some of the halachos and customs of Purim Meshulash, drawing on several sources.
As is well-known, Purim is celebrated in Jerusalem (and other cities that were walled at the time of Yehoshua), a day later than other cities, on the fifteenth of Adar – Shushan Purim.
This year, there will be a Purim Meshulash, as the fifteenth of Adar falls out on Shabbos. Residents of walled cities will fulfill the mitzvos of Purim over a three-day period: Kriyas Megillah and Matanos L’Evyonim are fulfilled on the 14th, like in unwalled cities. On Shabbos, Al HaNissim [and additions to Kriyas HaTorah] are said, and on Sunday, shalach manos are given out and the seudah is held.
Why do walled cities celebrate over a three-day period on Purim Meshulash rather than fulfill all the mitzos on Sunday? Kriyas Megillah is forbidden on Shabbos [lest one carries the Megillah in a public domain], and according to Chazal, since the Megillah says that the days of Purim “should not pass by,” it must be read earlier rather than later [unlike Tisha B’av for example, which is delayed to Sunday when falling out on Shabbos].
Matanos l’evyonim is fulfilled on the 14th and not delayed until Sunday since the Gemara states that matanos l’evyonim should be given on the day the Megillah is read because the poor expect it then and they’ll be able to benefit from it as early as possible.
Seudas Purim is delayed until the 16th and not fulfilled on Shabbos so as not to mix one simcha with another simcha. Giving out shalach manos, while not forbidden on Shabbos, can lead to chillul Shabbos [carrying], so it is fulfilled on Sunday as well. Also, most poskim hold that shalach manos should be given out on the 16th since it is the main Purim day, based on the Rambam, who hints that the seudah and shalach manos are interrelated.
However, some poskim hold that Yerushalmim should give out a small amount of shalach manos on Friday and Shabbos [if possible]. Other poskim say that guests at the table can fulfill the mitzvah of shalach manos.
Although Friday is not Purim in Jerusalem, it is customary to wear Shabbos clothes in honor of the Megillah reading as well as to serve extra food and wine at the Shabbos meals, making the seudos more special in honor of Purim.
(YWN Israel Desk – Jerusalem)