The fifteenth of Adar, when Purim is celebrated in walled cities, sometimes coincides with Shabbat. The fourteenth of Adar never coincides with Shabbat. When the fifteenth falls out on Shabbat, Purim in that year is called Purim Ha-meshulash (“Triple Purim”), because its mitzvot are divided over three days. Why don’t we celebrate Purim on Shabbat? The Sages prohibited us from reading the Megilla on Shabbat for fear that one may carry it through a public domain. It is also inappropriate to conduct the se’uda on Shabbat, because it says, “To observe them as (lit. ‘to make them’) days of feasting and joy” (Esther 9:22), which means that the meal must be made for the sake of Purim, not Shabbat. (The date of Purim depends on an act of the beit din, which sanctifies the new month, while Shabbat is set and established from the time the world was created.)
Therefore, residents of walled cities read the Megilla on Friday. They also give matanot la-evyonim on that day, because poor people anticipate receiving money at the time of the Megilla reading and will be distressed if people neglect to give them gifts on that day. One should try to read the Megilla with a minyan, because some maintain that this reading is not conducted in its proper time, in which case it must be read in a group of ten Jews, men or women (mb 690:61). Nonetheless, the Megilla should be read on Friday, with the berakhot, even when no minyan is available (Tzitz Eliezer 13:73, Yabi’a Omer 6:46).
On Shabbat, which is the day of Purim in walled cities, residents of these cities recite Al Ha-nisim in the prayer services and in Birkat Ha-mazon. They also read the special Torah reading for Purim. That is, they take out two Torah scrolls; they read the weekly Torah portion from the first scroll and the portion for Purim – “Amalek came” (Shemot 17:8-16) – from the second scroll.
On Sunday, they eat the festive Purim meal and send mishlo’aĥ manot to one another, since mishlo’aĥ manot is connected to the meal.
The reason we read the Megilla before Shabbat but postpone the se’uda until after Shabbat is because we must publicize the miracle, by reading the Megilla, no later than the day on which the miracle occurred, as it says, “ve-lo ya’avor (lit. ‘and it shall not pass’)” (Esther 9:27). On the other hand, we may partake in the se’uda only once the day of the miracle has arrived, which is Shabbat. And since it is inappropriate to make such a meal on Shabbat, we postpone it until after Shabbat.
It is best to eat more meat and drink more wine than usual even on Shabbat, because some authorities maintain that the se’uda is supposed to be eaten on Shabbat. If possible, it is also good to send mishlo’aĥ manot privately on Shabbat, on condition that there is an eruv in one’s location.
According to many poskim, a resident of an unwalled city who comes to Jerusalem on Friday does not need to observe the mitzvot of Purim Ha-meshulash, because he has already fulfilled his obligation. Others maintain that it is better for him to observe the mitzvot of Purim Ha-meshulash.