In the winter season, we mention rain twice in the Amidah. In the beginning, we mention rain in our praise of Hashem Who causes rain to fall; afterwards, we ask Hashem to bless us with dew and rain.
In the second berachah of the Amidah, Mechayei HaMeitim, we praise the One “Who makes the wind blow and the rain fall,” (“Mashiv haru’ach u’morid hagashem”). Chazal instituted mentioning the praise of rain in this particular berachah because rain gives life to the world.
In the ninth berachah, Birkat HaShanim, we request rain. According to the Sephardic minhag, the entire wording of Birkat HaShanim changes from winter to summer: in the winter we start with “Barech Aleinu” and in the summer we start with “Barcheinu Hashem Elokeinu.” According to the Ashkenazic minhag, the wording of Birkat HaShanim in the summer and in the winter is identical, with the exception of the words “v’ten tal u’matar” in the winter, and “v’ten berachah” in the summer.
Although both the mention of rain and the request for it are recited in the winter, there is a difference between them. The mention of rain is recited at the appropriate season for rain, whereas the prayer for rain is only recited when we actually want rain to start falling.
The mention of rain begins on the holiday of Shemini Atzeret. Conceivably, rain could be mentioned from the beginning of Sukkot, since from then on, the period of rain begins. However, because rain is considered to be a sign of a curse on the holiday of Sukkot, for we cannot fulfill the mitzvah of sitting in the sukkah when it is raining, we therefore do not mention it then. Chazal chose the Musaf prayer of Shemini Atzeret as the point at which we begin to mention rain, for then the whole community comes to synagogue and the announcement of its recital can be made in the presence of everyone. Ma’ariv was not chosen because not everyone comes to synagogue for the Ma’ariv prayer. Likewise, it cannot be announced before reciting the Amidah of Shacharit because we are prohibited from interrupting between redemption and prayer (Beit Yosef and Shulchan Aruch, Orach Chaim 114:1-2).
However, the beginning of the request for rain was delayed fifteen days to the seventh night of the month of MarCheshvan. This is so that the last of the people returning home to the area of the Euphrates River from a Sukkot pilgrimage to the Temple in Jerusalem, would arrive without encountering rain on their way (Shulchan Aruch 117:1). Even after the destruction of Jerusalem, this custom was not abolished, since any minhag that reminds us of the great days of the Temple is cherished. Only after the Temple is rebuilt will the Sanhedrin be able to determine whether or not to change the time that we start asking for rain, taking into consideration contemporary means of transportation.
We continue to mention rain until the first day of Pesach; in Shacharit we still mention rain, but in Musaf we start mentioning dew. Regarding the request for rain, since the request is only made on weekdays, it turns out that the last time we ask for rain is in Minchah of Erev Pesach.