1,372. The First Series of Fasts for Rain

Hilchos Taaniyos 3:1

Let’s say that no rain at all has fallen since the rainy season began. If 17 Marcheshvan arrives without rain, Torah scholars start to fast Monday, Thursday and Monday. Any Torah student is qualified to accept this commitment upon himself.

Hilchos Taaniyos 3:2

If Rosh Chodesh Kislev arrives without rain, the court calls for a series of three communal fasts, Monday, Thursday and Monday; it is permitted to eat and drink on the evenings of these dates. The kohanim on duty in the Temple do not participate in these fasts because they are involved with the service. On these days, everyone should go to the shul to pray, cry out to God and recite supplications as is the standard practice on fasts.