We now turn to Maseches Taanis, chapter 1, mishnah 3, which deals with the question of when we begin to request rain in our prayers - that is, from when we say "v'sen tal u'matar".
The text of the mishnah:
"Bishloshah b'Marcheshvan sho'alin es hageshamim" - this is the opinion of the Tanna Kamma: on the third of Cheshvan we begin to request rain by saying "v'sen tal u'matar".
"Rabban Gamliel omer, b'shivah bo" - in his view, only on the seventh of Marcheshvan do we begin to say "v'sen tal u'matar", which is "chamishah asar yom achar hechag" - fifteen days after the conclusion of the festival of Sukkos.
Rabban Gamliel's reason is stated explicitly in the mishnah: "kedei sheyagia acharon sheb'Yisrael li'nhar Peras" - so that the last of the pilgrims, leaving Eretz Yisrael on his way back to Bavel, should have time to reach the Euphrates River before the rains begin to fall.
The time in the diaspora:
Everything stated in the mishnah refers to Eretz Yisrael, and there we would indeed begin at these dates. The Gemara, however, cites a beraisa teaching that in the diaspora we begin saying "v'sen tal u'matar" sixty days after the season of Tishrei. The seasons are based on the times of the year, meaning the solar year and not the lunar calendar, and therefore we begin on the fourth of December - sixty days after the season of Tishrei; and in a leap year, on the fifth of December, and so on.
In summary: in this mishnah we learned the dispute between the Tanna Kamma and Rabban Gamliel regarding when rain is requested in Eretz Yisrael - the third or the seventh of Marcheshvan, and Rabban Gamliel's reason, that the last of the pilgrims should have time to reach the Euphrates River; and in contrast, the practice in the diaspora, sixty days after the season of Tishrei, beginning on the fourth of December.