As of tonight, which is Monday according to the secular calendar but already Tuesday (Yom Shlishi, the 7th of Cheshvan) in the Jewish calendar, Jews in Israel will start inserting the formal request for rain in the 9th blessing of the thrice-daily Amidah prayer. Thus will officially begin the rainy season in Israel, which will go on until the first day of Passover.
Even though Jews everywhere pray for rain on Shmini Atzeret (Tishrei 22), we don’t ask that it begin right away.
In Israel, this request is inserted on the 7th of Cheshvan, based on the view of Rabban Gamliel, who says prayer for rain should be suspended until fifteen days after the festival of Sukkot, so that the last Jewish pilgrims returning home to Babylon from Jerusalem will reach the River Euphrates and not be delayed by the rain and the muddy roads.
In the Diaspora, we wait until December 4th to start asking for rain, based the opinion of Chananiah who says that in the Diaspora we do not begin to pray for rain until the sixtieth day after the fall holidays.