1. There is a dispute about when we begin to recite Morid ha'Geshem.
2. Rebbi Yehoshua questions why we should say Morid ha'Geshem on Sukos if rain is a bad omen during Sukos.
3. Rebbi Yehudah: Morid ha'Geshem is first recited during Musaf on the last day of Sukos.
4. While Hashem makes angels do His bidding in many areas, three keys were not given to an angel, but remain only in the hands of Hashem.
5. Rain that comes after Nisan is a sign of a curse.
A BIT MORE
1. Rebbi Eliezer: We begin on the first day of Sukos. Rebbi Yehoshua: We begin on the last day of Sukos.
2. Rebbi Eliezer responds that Morid ha'Geshem is not a request for rain, but rather it is a praise saying that Hashem is the One Who brings down the rain.
3. It is said until Musaf on the first day of Pesach.
4. These are the keys of: rain (including Parnasah, which some list separately as a fourth key), giving birth, and raising the dead.
5. The verse states, "Isn't it the time of harvest of wheat?" This implies that it was a bad time for rain. (While this is generally true, circumstances vary for each country depending on how badly they need rain).