Taanis 2:8-9

Taanis 2:8

Whenever Megillas Taanis says that it is prohibited to eulogize on a certain day, it is also prohibited on the preceding day but it is permitted on the following day. Rabbi Yosi says that it is prohibited on both the preceding day and on the following day. When it says that fasting is prohibited on a certain day, one may do so on both the preceding day and the following day. Rabbi Yosi says that it is prohibited on the preceding day and permitted on the following day.

Taanis 2:9

Public fasts are not started on a Thursday in order not to raise the price of food in the marketplace. Rather, the first fasts are held on Monday, Thursday and the following Monday. The second fasts may follow on Thursday, Monday and Thursday. Rabbi Yosi says that just as the first fasts are not started on a Thursday, neither are the second fasts, or the last fasts.