Before us is Mishnah 8 in the second chapter of Tractate Taanis, which deals with an ancient work called Megillas Taanis, a composition that predates even the writing of the Mishnah.
What is Megillas Taanis:
It is a list of days on which special salvations occurred for the Jewish people. Because of those salvations, the Sages decreed that on some of these days it is forbidden to fast, and on some of them, in addition to the prohibition of fasting, it is also forbidden to deliver a eulogy, so that if a funeral falls on such a day, no words of eulogy are said.
The words of the Mishnah:
"Kol hakasuv b'Megillas Taanis d'la l'mispad - lefanav asur, l'acharav mutar. Rabbi Yosi omer: lefanav ul'acharav asur" - the Mishnah distinguishes between two types of days:
"D'la l'mispad" - a day about which it was stated that one may not deliver a eulogy on it. This is the more significant type of day, one whose level of rejoicing is greater. According to the first Tanna, the day before it is also forbidden in fasting and eulogy, while the day after it is permitted. Rabbi Yosi disagrees and holds that both the day before and the day after are forbidden in eulogy and in fasting.
Days on which only a prohibition of fasting was stated - these are not as significant as those days on which eulogy was also forbidden, and therefore, according to the first Tanna, it is permitted to fast both on the day before them and on the day after them. Rabbi Yosi holds that regarding these days too, the day before is forbidden in fasting, while the day after is permitted.
The practical halachah:
It is important to note that the halachah was decided that Megillas Taanis has been nullified, and those days are no longer observed in practice, except for two days alone, the ones we know as Chanukah and Purim. On these days eulogy and fasting are forbidden, but the days before them and after them are permitted.
In summary: In this Mishnah we learned about Megillas Taanis, the list of days on which salvations occurred for the Jewish people, and about the two types of days it contains: days on which eulogy and fasting were forbidden, and days on which only fasting was forbidden. We examined the dispute between the first Tanna and Rabbi Yosi regarding the law of the adjacent days, and the halachah that Megillas Taanis has been nullified except for Chanukah and Purim.