In Parshat Pinchas (28:16-29:39), the Torah tells us of the special Korban Musaf brought on each of the holidays. On each day of Sukkot, the number of cows decreased by one, starting with thirteen on the first day and ending with seven on the last. The total number of cows brought over the entire holiday is 70, a number associated with the number of nations in the world. Some explain that these cows were Korbanot for all the nations of the world. If the process of decreasing by one cow each day could be easily explained, as above, why does the Torah feel it necessary to mention the Korban of each day individually (2:17-34), using the same words over and over?
One answer, found in Masechet Taanit (2b), is based on the slight differences found in the context of some of the days. On the second day the word “Veniska” is changed to “Veniskeihem.” On day six, the same word is changed to “Unesacheha,” and on day seven the word “Kamishpat” is changed to “Kimishpatam.” The three letters that appear to be extra spell the word מים, water.
רבי יהודה בן בתירא says, based on the change made on the second day, that one should begin saying “Mashiv Haruach Umorid Hageshem,” the prayer that states God’s power over rain, starting on the second day, since the beginning of the hint to water appears here. Rabbi Akiva says we start on the sixth day, due to its extraneous letter. The extra Yud of the sixth day changes the word “Veniska,” “and its libation” to “Unesacheha,” “and its libations.” Rabbi Akiva interprets the plural to refer to the water libation brought on Sukkot together with the regular wine libation. Since he believes that this is the hint to that additional libation, this day is the first on which we say Mashiv Haruach.
Sukkot, it seems, is really a holiday about water. The rainy season in Eretz Yisrael begins around this time, the water libation was brought, and the Simchat Bait Hashoeva was celebrated on Sukkot. One can even say that the Mitzva of Arba Minim requires rain, so that the different plants can grow.
In addition, this author suggests an alternate usage of the water relationship, relating back to the question of why such similar Pesukim are repeated many times. Water is often equated with Torah. Just as water is a source of life, Torah is the source of life. Just as one is unable to go far without water, one will not get anywhere without Torah. What this analogy teaches us is to guard the Torah carefully, to learn it carefully, and to keep the Mitzvot.