תענית ב׳:ח׳ - ב׳:ט׳

כל הכתוב במגילת תענית – days in which miracles were performed for Israel during the Second Temple period and they established holidays. There is a day where one only does not fast on them and there are days in which they were more stringent by not giving eulogies on them, and those in which they were more stringent, it was forbidden to give eulogies.

לפניו אסור – to give eulogies [on the day before], lest they come to do this on the holy day itself [where it was absolutely forbidden].

לאחריו מותר – since the holiday passed, we are not concerned. And we establish for ourselves [today] since the Scroll of Fasts has been voided, that all of the days that were written there to not fast or to give eulogies on them – all of them are permissible [now] to fast and give eulogies [on them], except for Hanukkah [25 Kislev to 2/3 Tevet] and Purim [i.e. 14 Adar or 15 Adar in a walled city from the time of Joshua] , which in which giving eulogies and fasting are prohibited, but on the days before them and after them, it is permissible.

שלא להפקיע את השערים – When we see that store owners would purchase two large meals at the conclusion of the fifth day of the week (i.e., Thursday), one for the night of the fast and one for Shabat, people would think that there a famine is coming in the world causing prices to rise and unsettling the market. But when they started to fast on Mondays, people knew that it was none other than because of [the need to] fast [for rain to fall]. Another explanation: that they should not say that if the Jewish court did not see that the famine is coming, they would not have pushed to decree a fast now so close to Shabbat, and because of that, the prices would rise and the market became unsettled.