Midrasz do Wyjścia 34:22

This month shall be unto you (Exod. 12:1). R. Ishmael said: He showed him the moon during night and said: Like this shall you examine the new moon; this shall be the law for future generations. Teach them the time of the new moon. He said to them: Until now I intercalated the year (adding an additional month in leap years), but now I turn this responsibility over to you. From now on you shall count (the days and designate the time) for the appearance of the new moon. This month. This is Nisan, and there is no other (i.e., no intercalated second Nisan to sere as the first month). With regard to Tishri it is written: And the feast of ingathering (Sukkot) at the turn of the year (the autumnal equinox) (Exod. 34:22). This teaches us that the month in which the festival called the feast of the ingathering at the turn of the year occurs is the month in which the year actually changes. However, Tishri is called the seventh month only because it comes seven months after Nisan. This month refers to Nisan. A hint of this (is the fact that it is the only month that is spoken of as the first month),⁷Nisan begins the secular year and therefore is counted as the first month. It marks the anniversary of the deliverance from Egyptian bondage. It is the month in which the reigns of kings begin, and the month from which the festivals are instituted. as Scripture states: In the first month, which is the month of Nisan (Est. 3:17), first for months, and first for festivals, since (Passover) is written first: The festival of Passover, the festival of Shavuot, the festival of Sukkot.

The great cycle of the sun is 28 years, and therein are seven small cycles each of four years. The number of days of the solar year is 365 and a quarter of a day. The seasons of the solar year are four, each season (consisting of) 91 days 7½ hours. The beginnings of the cycles of the seasons are the 4th, 2nd, 7th, 5th, 3rd, 1st, and 6th (days). Between each cycle there are 5 days and 6 (hours).

8) And whence is it derived that they (the two loaves) precede the (bringing of) the first-fruits (bikkurim)? From (Shemoth 34:22): ("And the festival of Shavuoth shall you make for yourself,) the first of the wheat harvest" (i.e., What you make on Shavuoth [the two loaves] should precede [all that comes of] the wheat harvest.) This tells me (that it precedes only bikkurim) of wheat. Whence do I derive (that it precedes also bikkurim) of barley? From (Shemoth 23:16): "which you sow" (implying all that you sow.) This tells me only of what is sown. Whence do I derive (that the two loaves precede) what grows of itself (from seeds scattered by the wind)? From (Shemoth 23:16): "in the field" (implying all that grows in the field.) This tells me only of what grows in the field. Whence do I derive the same for (fruits planted on) a roof, in a yard, or in a ruin? From (Bamidbar 18:13): (The two loaves shall be) "the first-fruits of all that is in their land." And whence is it derived that they precede the (bikkurim of) libations and the fruits of the tree? From (Bamidbar 23:16): (The two loaves shall be) "the first fruits of your labor," and (Bamidbar 23:16): "when you gather your labor from the field." (Grapes for libations and fruits are subsumed in "gathered.")