He shall remove its crop with its feathers, and cast it beside the altar (1:16)
The bird flies about and swoops throughout the world, and eats indiscriminately; it eats food obtained by robbery and by violence. Said G‑d: Since this crop is filled with the proceeds of robbery and violence, let it not be offered on the altar. . . . On the other hand, the domestic animal is reared on the crib of its master, and eats neither indiscriminately nor of that obtained by robbery or by violence; for this reason the whole of it is offered up.
(Midrash Rabbah; Rashi)
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On the Shabbat that falls on or before the 1st of Nissan, a special reading called "Hachodesh" (Exodus 12:1-20) is added to the regular Shabbat Torah reading. Hachodesh recounts G-d's historic communication to Moses in Egypt on the 1st of Nissan (2 weeks before the Exodus) regarding the Jewish calendar, the month of Nissan and the Passover offering.
This Shabbat is Shabbat Mevarchim ("the Shabbat that blesses" the new month): a special prayer is recited blessing the Rosh Chodesh ("Head of the Month") of the upcoming month of Nisan, which occurs tommorow (Sunday).
Prior to the blessing, we announce the precise time of the molad, the "birth" of the new moon. See molad times.
The months of the year are lunar months, as it is written: "This chodesh shall be for you the head of months." So said our sages: G‑d showed Moses the figure of the [new] moon in a prophetic vision, and said to him: "Thus you should see and sanctify."
However, the years which we figure are solar years, as it is written: "Keep the month of spring" [i.e., ensure that the month of Passover is always in the spring season].
The solar year is eleven days longer than a year of [twelve] lunar months. Therefore, when this surplus accumulates to the amount of 30 days—either a little more or a little less—they add an extra month, so that the year has 13 months; this is what is called a shanah meuberet ("pregnant year"). [This is done] because one cannot make the year to consist of so many months plus so many days, since the verse says, "[It shall be for you the first of] the months of the year"—implying that the year should consist of months, and months only.
The moon is concealed each month, and remains invisible for approximately two days—for about one day before it is closest to the sun, and about one day after it is closest to the sun—after which it can be seen in the west in the evening. The night on which it is visible in the west marks the beginning of the month, and one counts from that day 29 days. If the moon is visible on the eve of the 30th, then the 30th day is Rosh Chodesh ("head of the month"); if not, then the 31st day is Rosh Chodesh, and the 30th day belongs to the previous month.
(Mishneh Torah, Laws of the Sanctification of the Moon)
The months of the year are lunar months, as it is written: "This chodesh shall be for you the head of months." So said our sages: G‑d showed Moses the figure of the [new] moon in a prophetic vision, and said to him: "Thus you should see and sanctify."
However, the years which we figure are solar years, as it is written: "Keep the month of spring" [i.e., ensure that the month of Passover is always in the spring season].
The solar year is eleven days longer than a year of [twelve] lunar months. Therefore, when this surplus accumulates to the amount of 30 days—either a little more or a little less—they add an extra month, so that the year has 13 months; this is what is called a shanah meuberet ("pregnant year"). [This is done] because one cannot make the year to consist of so many months plus so many days, since the verse says, "[It shall be for you the first of] the months of the year"—implying that the year should consist of months, and months only.
The moon is concealed each month, and remains invisible for approximately two days—for about one day before it is closest to the sun, and about one day after it is closest to the sun—after which it can be seen in the west in the evening. The night on which it is visible in the west marks the beginning of the month, and one counts from that day 29 days. If the moon is visible on the eve of the 30th, then the 30th day is Rosh Chodesh ("head of the month"); if not, then the 31st day is Rosh Chodesh, and the 30th day belongs to the previous month.
(Mishneh Torah, Laws of the Sanctification of the Moon)
Time is the first creation (see Sforno on Genesis 1:1); thus, the sanctification of time is the first mitzvah commanded to Israel.
(The Lubavitcher Rebbe)
