Purim in Synagogue
Purim itself begins with the recitation of the regular Evening Service. As on Hanukkah, (also spelled Chanukah) a version of the Al Ha-nissim prayer is added in the penultimate blessing of the Amidah and in the Grace after Meals.
The morning Torah reading (Exodus 17:8–16, divided into three aliyot) describes the attack of Amalek.
Tradition dictates that Purim be observed on the fourteenth day of Adar.
However, in any city that was surrounded by walls in the days of Joshua, the festival is observed a day later on the fifteenth of Adar (Esther 9:17–18, as interpreted in M M’gillah 1:1).
Outside of Israel there are no cities with Jewish communities that meet this requirement.
But Jerusalem, even today, is deemed to be in that category and therefore Purim is observed there on the fifteenth of Adar, called Shushan Purim.
Reading the Megillah
Some other cities are of ambiguous status, and thus many communities in Lod, Tiberias, and Safed read the megillah on both the fourteenth and the fifteenth.
While the calendar is arranged so that Purim can never fall on Shabbat, it is possible for Shushan Purim to fall on Shabbat—which creates a three-day commemoration, Friday being Purim, and Shabbat being Shushan Purim with respect to some prayers but not with respect to the reading of the megillah, which is deferred to Sunday.
The Absence of Hallel
Surprisingly, Hallel is not recited on Purim.
The Talmud (at BT Megillah 14a) suggests that the salvation that came to the Jews on Purim is of a more limited nature than the salvation of Passover, where the Jews were not only saved from imminent destruction but also delivered out of Egypt and out of the hands of Pharaoh.
Such is also the case on Ḥanukkah: the Jews were not only saved from imminent destruction but also delivered from the tyranny of Antiochus.
In the Purim story, on the other hand, while the Jews were delivered from imminent danger, they were specifically not delivered from Ahasuerus’s dominion, nor were they freed from any future threats he might yet have posed to their well-being.
As a result, Hallel is not recited.
It is also worth noting Maimonides’ opinion, codified in the Mishneh Torah, that there actually is an obligation to recite Hallel on Purim (presumably because the Jewish people of Persia were indeed saved from destruction), but that the reading of the megillah serves as Purim’s version of Hallel (Mishneh Torah, Hilchot Megillah vChanukah 3:6).
Adapted with permission from The Observant Life.