The might of the rains is mentioned:

(R. Eliezer): From the first day of Sukos.

(R. Yehoshua): From the last day of Sukos.

Since rain on Sukos is a bad sign, why mention it?

(R. Eliezer): I did not say to ask for it, only to mention that it happens at its proper time.

(R. Yehoshua): If so, it should be mentioned all year!

We only ask for rain close to its time.

(R. Yehudah): On the last day of Sukos, the Shali'ach Tzibur for Musaf mentions rain, but not that of Shacharis.

On the first day of Pesach, the Shali'ach Tzibur for Shacharis mentions rain, but not that of Musaf.

Question: Where have we learned that there is an obligation to mention rain, leading the Mishnah to discuss when this begins?

Suggested answer: In Maseches Berachos, we learned that the might of the rains is mentioned in the Berachah of the Resurrection of the Dead.

Question: Why was not the date of when it begins discussed there?

New answer: Our Tana is continuing from Rosh Hashanah, where we learned that on Sukos, judgment is passed for water.

Question: Why does the Mishnah speak of "the might of" the rains?

Answer: Because they descend with might, as we see from the following Derashah:

A Pasuk describes the rain as being "without comprehension" and another Pasuk uses the same term to describe the creation of the world, which is also described as being done with might.

Question: How do we know that this should be mentioned during prayer?

The Pasuk states that we are to serve Hash-m with our hearts, which refers to prayer, and it is followed by a reference to the rain.

(R. Yochanan): Hash-m has three keys that are not entrusted to an intermediary - those of rain, childbirth, and resurrection of the dead, as derived from Pesukim.

In Eretz Yisrael, they added: also the key of sustenance.

R. Yochanan held that that was included in rain.

The Mishnah quoted R. Eliezer that mention of rainfall begins on the first day of Sukos.

Question: Did he learn it from Lulav, and like Lulav, it begins by day, or did he learn it from Nisuch haMayim, and it begins from the evening.

Answer: R. Avahu said that he learned it from Lulav.

Some say he received this as a tradition, and some say he derived it from the following Beraisa:

From when do we mention the rains?

(R. Eliezer): From the day that Lulav is taken.

(R. Yehoshua): From the day that the Lulav is put away.

(R. Eliezer): The Four Species are taken as an appeasement for water; like them, the world cannot survive without water (and therefore we should mention the rains on Sukos.)

(R. Yehoshua): But rain on Sukos is a bad sign!

(R. Eliezer): I did not say to ask for rain, only to mention it, just as Techiyas ha'Meisim is mentioned all year yet only happens at its proper time; therefore, if a person wants to mention the rains all year, he can.

(Rebbi): From the day that we stop asking for rain, we should also stop mentioning it.

(R. Yehudah b. Beseirah): The rains are mentioned from the second day of Sukos.

(R. Akiva): They are mentioned from the sixth day of Sukos.

(R. Yehudah, citing R. Yehoshua): On the last day of Sukos (Shemini Atzeres), the Shali'ach Tzibur for Musaf mentions the rain, but not that of Shacharis; on the first day of Pesach, the Shali'ach Tzibur of Shacharis mentions the rain, but not that of Musaf.

Question: What is R. Yehoshua's response to R. Eliezer's argument?

Answer: Techiyas ha'Meisim would be welcomed at any time, but a Mishnah teaches that rain after Nisan is a bad sign.

Question: What is the reason of R. Yehudah b. Beseirah?

Answer: The letters of MaYiM are spelled out in the Torah, beginning on the second day, alluding to the Nisuch ha'Mayim.

R. Akiva's reason is that the Pasuk of the sixth day alludes to two Nisuchim, that of wine and that of water.

Question: Perhaps they both refer to wine?

Answer: R. Akiva holds of R. Yehudah b. Beseira's acronym of MaYiM.

Question: Why does he not also hold that it begins on the second day?

Answer: As the superfluous mention of Nisuch is on the sixth day.