Tossefta sur Les Psaumes 68:10

The [sacrificial] bulls of Sukkot are no fewer than seven nor greater than fourteen. There is no re-opening [of the niddah count for a woman who had been mistaken in her reckoning] fewer than seven nor greater than seventeen [days] (Arakh. 2:1). There is no leprosy of less than one week nor greater than three weeks. There are no fewer than four full (i.e., 30-day) months in a year, nor does it appear proper to increase [the number of full months] to greater than eight (Arakh. 2:2). It never appeared proper to have six full months one year after another. In a place where they do not know what date [is the first] of the month, they are accustomed for one [month] to be full and for [the next] one not to be full, and so too in the [lands of] exile, they are accustomed for one to be full and for [the next] one not to be full. They had been accustomed for Tammuz to be in its time (i.e., 29 days) and for Av to be full, and afterwards it was made known to them that Av is in its time. They left it as is (i.e., they did not deduct a day retroactively), and they started counting from that point forward one month full and one month not full. Sometimes Shavuot falls on the fifth, or on the sixth, or on the seventh [of Sivan], never earlier and never later. Rabbi Yehuda says, if it falls on the fifth, that is a bad sign; on the sixth, it is always an in-between sign; on the seventh, it is always good sign. [Abba Shaul says, whenever Shavuot falls on a clear day, it is a good sign (MS Vienna).] And so too it comes out that Israel was standing before Mount Sinai, they were pushed backwards twelve mil, and brought forwards twelve mil, with every single word, twenty-four mil. That was a clear day, and Israel were being cast away because of the fire. The Holy One Blessed be He said the Clouds of Glory, "Let dew fall upon My children," as it is said (Judges 5:4), "When God came forth from Seir, advanced from the land of Edom, [the earth trembled, even the heavens dripped, and even the clouds dripped water]." And the Holy One Blessed be He let rain and dew fall before them, as it is said (Ps. 68:10), "A bountiful rain You sent in waves (tanif), O God, etc." Shavuot only falls on the day of nof^*ביום הנף; this could be variantly translated as "a day of waving," as in the day of the Omer; or "a day of panoramic views," as in a clear day, connecting with Abba Shaul's dictum. Rosh Hashanah only falls on a day of nof or on the day of its fullness (i.e., the next day if the previous month had 30 days). Others say, there can only be four days' difference between one Shavuot and the next Shavuot, and there can only be four days' difference between one Rosh Hashanah and the next Rosh Hashanah, and if it was a leap year, then five days.