Taanis 2:10-3:1

Taanis 2:10

Public fasts are not decreed for Rosh Chodesh, Chanukah or Purim but if a series of fasts has been already begun, it is not interrupted; this is the opinion of Rabban Gamliel. Rabbi Meir says that even though Rabban Gamliel says that the fast series is not interrupted, he agrees that on these days one would not fast all day, the same as when Tisha b’Av falls on Friday. [The way we have the calendar arranged, Tisha b’Av cannot fall on Friday.]

Taanis 3:1

The order of fasts that we have been discussing applies to the first rains but if plant growth is affected, we cry out immediately. Similarly, if there is an interval of forty days in between rainfalls, we cry out immediately because this is a drought.