We do not request rain except close to the rainy season.  R' Yehudah says: Of those [chazzanim] who stand before the Ark on the last Yom Tov of the festival —- the last [chazzan] mentions [the powers of rain, but] the first [chazzan] does not mention [it]. On the first Yom Tov of Pesach, the first [chazzan] mentions [the powers of rain, but] the last [chazzan] does not mention [it].  Until when do we request rain? R' Yehudah says: Until Pesach is over; R' Meir says: Until Nissan is over, as it is said: He caused rain to fall for you —- the early rain and the late rain —- in the first month.