Tractate Ta’anit, as its name indicates, focuses mainly on the halakhot and themes of fast days [ta’aniyot], covering both communal fasts and private, individual fasts and addressing both those that have fixed dates and those that are established from time to time in response to various negative events. In the Jerusalem Talmud and certain works of the early authorities, the tractate is called by the plural term, Ta’aniyot.
Tractate Megilla primarily focuses on explaining all of the halakhot that apply to the holiday of Purim. Additionally, it devotes considerable space to the general halakhot of synagogues and Torah readings. The holiday of Purim and its laws are mentioned in the Scroll of Esther but not in the Torah itself, as the events that the holiday commemorates occurred hundreds of years after the giving of the Torah. Although there is a biblical basis for this commemorative holiday, based upon the mitzvot of remembering and defeating Amalek, Purim has a halakhic status in between those of Torah law and rabbinic law, and some of its mitzvot are treated as purely rabbinic enactments.