as the rabbis decreed that in certain places one may read the Scroll of Esther on the 11th, 12th, or 13th of Adar, nevertheless, it is permitted to eulogize and fast on these days. The Talmud clarifies: When does this ruling apply? If we say that it applies to those in walled cities, who normally read the scroll on the 15th of Adar and yet this year they read it on the 14th, a day on which they normally are permitted to fast and eulogize, but this cannot be the case, as are they permitted to fast and eulogize at all on these days?
והכתיב במגילת תענית: יום ארבעה עשר בו ויום חמשה עשר בו, יומי פוריא אינון דלא למיספד בהון. ואמר רבא: לא נצרכא, אלא לאסור את של זה בזה ואת של זה בזה.
But isn’t it written in Megillat Ta’anit: The day of the 14th of Adar and the day of the 15th of Adar are the days of Purim, on which eulogizing is prohibited. And Rava said: Since these days are already mentioned in the Bible (Esther 9:18–19), it is necessary to state this halakha in Megillat Ta’anit only to prohibit those living in these walled cities from fasting and eulogizing on this date, the 14th, and those living in these non-walled cities from fasting and eulogizing on this date, the 15th.
ואלא, בני ארביסר וקא קרי ליה בתליסר — יום ניקנור הוא! ואלא, בני ארביסר וקא קרי ליה בתריסר — יום טוריינוס הוא!
The Talmud continues its explanation of the difficulty. But rather, the Mishnah must be referring to those who normally read on the 14th of Adar, but who read the Scroll of Esther early, on the 13th. However, it is already prohibited to fast on the 13th, as it is Nicanor’s Day, which is a commemorative day in its own right. But rather, you will say that the Mishnah is referring to those residents of cities who normally read on the 14th, but who read it early that year, on the 12th; however, the 12th of Adar is also a commemorative day, as it is Trajan’s Day.