Taanis 1:3

Prayers for rain are recited on the third of Marcheshvan; Rabban Gamaliel says on the seventh, which is 15 days after Succos. This is enough time for the last departing pilgrims to reach the Euphrates river.

Taanis 1:4

If 17 Marcheshvan arrives and it still hasn't rained, individuals start keeping three fasts. On the night before, they may eat and drink, and on the fast day itself they may work, bathe, anoint, wear leather shoes and engage in marital relations.