As indicated by Jewish prayer, the rainy season in the land of Israel begins just after the holiday of Sukkot and ends at the start of Passover. During this time, there are two special phrases that are added to the Amidah, the central prayer of each service. The first - Mashiv ha'ruach u'morid ha'geshem, Who causes the wind to blow, and the rain to fall - is an expression of praise added to the second blessing of the Amidah beginning with the Mussaf service on Shemini Atzeret. The second insertion - v’tein tal u’matar, And bestow dew and rain for blessing* - is a request added to the ninth blessing of the Amidah (the blessing for a year of economic well-being). But, the date on which this phrase is added depends on where one lives. Those residing in the land of Israel begin saying V’tein tal u’matar on the 7th of Cheshvan (today). Those living outside of Israel, however, add it at the beginning of December.**
Whereas mashiv ha'ruach focuses specifically on rain for the land of Israel, v’tein tal u’matar is connected to one’s individual success, which is why it is included in the request for a year of economic well-being. The sages therefore took into consideration the well-being of the individuals and set the date for beginning to pray for rain on the 7th of Cheshvan, two weeks after the end of the holiday, so that the farmers who came to Jerusalem would be able to get home before the rains begin.
At the same time, the sages were concerned with the needs of the great number of Jews who lived in Babylon. Following Babylon’s seasonal schedule, the sages set the time to recite v’tein tal u’matar 60 days after the halachically recognized fall equinox. While the diaspora has expanded to encompass the entire world, the date for those living outside the land of Israel remains the one set for Babylon. This year, that date correlates to December 5th on the Gregorian calendar.
* The rest of the year, one recites - v’tein bracha And bestow blessing - at this same point in the prayer
**Due to the calculations of the Gregorian calendar, this date shifts by one day every century or so.