Taanit, Chapter Two, Mishnah Eight
In mishnaic times there existed a scroll called Megillat Taanit, which literally translates as Scroll of Fasts. Despite its name, the scroll does not contain a list of fasts but rather a list of days on which it is forbidden to eulogize at funerals and/or fast. Most of these days were commemorations of various military victories that occurred throughout the Second Temple period, from the Hasmonean period in the mid-second century B.C.E. through the Roman period in the subsequent centuries. By Talmudic times these days were mostly forgotten and neglected. The only two that were still observed were Hannukah and Purim.
1) [With regard to every day] about which it is written in the Scroll of Fasts (Megillat Taanit)One may not eulogizeon the preceding day it is prohibited but on the following day it is permitted.
a) Rabbi Yose says: it is forbidden [to mourn] both on the preceding day and on the following day.
2) [On days about which it is written], One may not fast on the preceding day and on the following day it is permitted.
a) Rabbi Yose says: on the preceding day it is forbidden but on the following day it is permitted.
Section one: Concerning some of the days listed in Megillat Taanit, it says that one may not eulogize because eulogies are a sign of mourning. For these days it is prohibited to eulogize also on the previous day, but not on the subsequent day.
Rabbi Yose holds that for these days one may not offer a eulogy on the preceding or on the following day.
Section two: Days concerning which it is written One may not fast are somewhat lesser than days concerning which it is written One may not eulogize. Therefore, for these days one may fast on both the preceding and following days.
Rabbi Yose is again slightly stricter and allows fasting only on the following day but not on the preceding day.