During the winter season, we mention rain twice in the Amida. At the beginning, we invoke rain to praise God, Who causes rain to fall; afterwards, we ask God to bless us with dew and rain.
In the second berakha of the Amida, Meĥayei Ha-meitim, we praise the One “Who makes the wind blow and the rain fall” (“Mashiv ha-ru’aĥ u-morid ha-gashem”). The Sages instituted this praise in this particular berakha because rain gives life to the world.
In the ninth berakha, Birkat Ha-shanim, we request rain. According to Sephardic custom, the entire wording of Birkat Ha-shanim changes from winter to summer. According to Ashkenazic custom, the wording of Birkat Ha-shanim in the summer and in the winter is identical, with the exception of the words “ve-ten tal u-matar livrakha” (“give dew and rain as a blessing”) in the winter and “ve-ten berakha” (“give blessing”) in the summer. (Yemenite Jews have an altogether different text, though the halakhic rule is the same.)
Although both the invocation and request of rain are recited in the winter, there is a difference between them. We invoke rain throughout the rainy season, whereas we request it only when we actually wish for it to begin falling.
We begin invoking rain on Shemini Atzeret. Technically, it could be invoked from the beginning of Sukkot, which coincides with the beginning of the rainy season. However, rain on Sukkot is considered an inauspicious omen since it prevents us from fulfilling the mitzva of sitting in the sukka. Therefore, we do not invoke rain then. The Sages therefore chose Musaf of Shemini Atzeret, when the entire community is in the synagogue, as the moment that it is announced to everyone that we begin invoking rain. Ma’ariv was not chosen because not everyone comes to the synagogue for Ma’ariv, and Shaĥarit was not chosen because the announcement may not be made right before the Amida, as we may not interrupt between redemption and prayer (Beit Yosef and SA 114:1-2).
The time to begin requesting was postponed for fifteen days to allow those who made the pilgrimage from the area of the Euphrates River to the Temple in Jerusalem for Sukkot to return home without being rained upon. Thus, we begin requesting rain at Ma’ariv on the seventh night of the month of Marĥeshvan (SA 117:1). Even after the destruction of the Temple, this custom was not abolished, since any practice that reminds us of the glory 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 requesting rain, taking into consideration contemporary means of transportation.
We continue to invoke rain until the first day of Pesaĥ, when we still invoke rain in Shaĥarit but begin invoking dew in Musaf. Regarding the request for rain, since the request is only made on weekdays, the last time we request rain is Minĥa of Erev Pesaĥ.