http://www.lookstein.org/
We begin requesting rain in the Diaspora on the 60th day of the fall
season, as approximated by Shmuel in the Talmud (Taanit 10a, Eiruvin 56a).
This year we began inserting V’ten Tal U’matar l’vracha in our Amida the night of Tuesday, December 4th.
For more information about this calculation, follow the link above., to a
fascinating article giving a (very brief) introduction to the Jewish
calendar, followed by a discussion on why we begin praying for rain when we
do (thanks to my friend Ari Brodsky for providing)