Excerpted from Or Hadash: A Commentary on Siddur Sim Shalom for Weekdays by Reuven Hammer
This phrase is found in 1 Kings 17:1 as part of Elijah's curse of the wicked King Ahab: “...there will be no dew or rain except at my bidding.” The presence of dew and rain would be the very opposite, a great blessing. In the summer, when there is no rain in the Land of Israel, we ask only for blessing. Since there is dew in the summer – on the first day of Pesah we recite a special prayer for it – Louis Ginzberg suggested that originally the phrases were תן טל לברכה, “grant dew as a blessing” (for the summer), and תן מטר לברכה, “grant rain as a blessing” (for the winter).
The Mishnah (Taanit 1:3) sets the date of 7 Heshvan, fifteen days after the conclusion of Sukkot, as the time to begin to pray for rain. This was intended to give pilgrims time to return from Jerusalem to their homes in Babylonia before the beginning of the rain. The Talmud (Eruvin 56a) records that in the Diaspora (Babylonia) the practice was to determine the date for this prayer according to the Equinox of Tishrei (autumn). It is recited sixty days after the Equinox (that is, usually from December 5th), since rain before that time would damage the date crops. When there is a civil leap year, however, the date is delayed until December 6th. This occurs whenever the Hebrew year is divisible by four (Lasker). This is the current practice throughout the Diaspora. Abudarham records that in Provence they followed the Hebrew calendar (7 Heshvan) and remarks "that seems very proper to me."