If the Seudah Shlishis on Shabbos Erev Purim (this year 27 Feb) stretches until after nightfall, one does not say Al Hanisim in Birchas Hamozon even though it is already Purim. [One does say Ritzay for Shabbos, as always]. Magen Avraham 419:1, Shulchan Aruch Harav 188:7, Nitay Gavriel 75:9:18
Ta’anis Esther, (this year moved up to Thursday 25 Feb) is a fast day for men and women. The fast day is to remember that Hashem listens when one is in a time of danger and need, as he did at the time of Purim. The fast is also a segulah to prevent the Satan from prosecuting an inadvertent sin done on Purim. Finally, many Mikubalim write that the decree against us from Haman has a potential reoccurrence each year and the fast day is a defense against the decree. Shulchan Aruch w/Mishnah Brurah 686:1, Magid Maysharim par. Vayakhel, S’V Shevet Hakahasi 1:203
Some people have the custom to learn Mishnayos Meseches Shekalim on Shabbos Parshas Shekalim after Mincha. (And Mes. Megillah on Shabbos Parshas Zochor, and Mes. Parah on Shabbos Parsha Parah, and Mes. Pesachim on Shabbos Parshas Hachodesh). Nit’ay Gavriel 17:11
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