Translation:Arukh ha-Shulchan/Orach Chaim/686

Section 1
The Shulchan Aruch and Tur write that on Hanukkah and Purim- it is permitted to fast (on the day) before (Purim) and (on the day) after (Purim). So are there words.

In other words: Even though we rule that Hanukkah and Purim are (exceptions to the rule) in the sense that (they are not nullified like the other holidays) mentioned in Megilat Taanit, it is written by (all holidays mentioned in) Megilat Taanit, that (the day) before and (the day) after them- it is prohibited to fast. However, because when (the Sages nullified) Megillat Tannit, they also nullified the prohibition of fasting before and after these holidays. (This nullification of the prohibition of fasting before and after a holiday mentioned in Megilat Taanit, did not include the exception of Hanukkah and Purim. Therefore, it is also permitted to fast the day before and the day after them.)

Although some argue (concerning whether one may fast the day before and after) Hanukkah, this is not the case concerning the Fast of Esther, which was universally accepted throughout the Jewish People. It says in the beginning of Tractate Megillah (2a):
 * The thirteenth (of Adar), is a time of the congregation for everyone.

Rabbenu Tam explains : (the reason for this is) because (it is incumbent upon) everyone to fast on Taanit Esther (the Fast of Esther). The villagers should come to the larger towns to say (with the members of the larger towns) supplements of forgiveness (סליחות), and pleas for forgiveness (תחנונים), in order to beg of (God) for their lives, and to beg for mercy. (Rash, Ran). However, Rashi argues and explains this passage of the Talmud (in the following way): When they assemble to