Q. Why do we use a civil date (4 or 5 December) for the commencement of saying v’ten tal umatar in the weekday Amidah, but a Hebrew date (Sh’mini Atzeret) for the commencement of mashiv haru’ach umorid hagashem?
A. There are two different practices related to rain:
1. V’ten tal umatar (“Grant dew and rain”), which prays for rain in the rainy season.
The Mishnah requires this prayer in the land of Israel from 7 Cheshvan, so that those who had been in Jerusalem for Sukkot would have time to get home before the rains began (Ta’anit 1:3).
The Talmud adds that in the Diaspora the prayer commences 60 days after the autumn equinox (Ta’anit 10a).
Why a later date outside Israel? According to Rashi, it is because less rain is needed there.
The equinox is related to the solar calendar, not the lunar, which is why the date is calculated as 4 or 5 December.
2. Mashiv haru’ach umorid hagashem (“He makes the wind blow and the rain fall”), which commences at the end of Sukkot on Sh’mini Atzeret.
This is not a request for rain but a praise of the Almighty who is the Giver of the rain.
It is inserted in the early part of the Amidah as one of the unique g’vurot (“powers”) of God.