Midrash su Deuteronomio 16:1

(Fol. 9a) R. Abba said: "All agree that the deliverance of Israel out of Egypt took place in the evening, as it is said (Deu. 16, 1.) Did the Lord, thy God, bring thee forth out of Egypt by night; and [they also agree] that they did not leave [Egypt] before the next morning, as it is said (Num. 33, 3.) On the morrow after the Passover sacrifice, the children of Israel went out with a high hand, but they differ as to the meaning of Chipazon (In haste, Ex. 12, 11). R. Elazar b. Azariah holds that Chipazon (In haste), refers to the Egyptians who [after realizing the plague of the first-born] hurried the children of Israel to leave; and R. Akiba holds Chipazon (In haste), refers to the Israelites [who were in haste to leave]." We have also a Baraitha to the same effect: Did the Lord, thy God, bring thee forth out of Egypt by night (Deu. 16, 1). "Did they really go out at night? Behold! it was in the morning, for it is said (Num. 33, 3.) On the morrow after the Passover sacrifice, did the children of Israel go out with a high hand. We must therefore say that the beginning of the deliverance was in the evening."

(Exodus 12:2) "It is the first": What is the intent of this? It is written (Devarim 16:1) "Observe the month of Aviv (spring) and offer the Pesach to the L rd your G d, etc." Observe Pesach for Aviv, and Aviv for Pesach, that Aviv fall out in its proper time. How so? Intercalate Adar, so that Aviv fall out in its proper time. If they intercalated Adar and Aviv did not fall out in its proper time, I might think that they should intercalate the New Moon of Nissan. R. Yishmael says: If you say this, you make Nissan twice; but the Torah has said: "It is the first (i.e., one) to you" (and not two). R. Nathan says "Observe the month of Aviv" — Observe the month which is closest to Aviv. And which is that? Adar. But we have not heard how many (days) are to be intercalated. From "Observe the month (of Aviv"), we infer that a month (of days, i.e., thirty) are to be intercalated. And this follows, viz.: A month is intercalated and a year is intercalated. Just as a month, (with) one of its units (i.e., days), so, a year, with one of its units (i.e., months). __ But perhaps, just as (with a) month — one thirtieth of it, so, (with) a year, one thirtieth of it? It is, therefore, written "Observe the month." (A month is intercalated and not one thirtieth of it. __ But perhaps, just as a year, one twelfth of it, so, a month, one twelfth of it? It is, therefore, written (Numbers 28:17) "And on the fifteenth day of this month" — You intercalate a day for a month, and not one twelfth of it. R. Yitzchak said: If you said this, the moon would already be in the middle of the sky (in the beginning of the month)! You must perforce revert, then, to the first formulation, viz.: A month is intercalated and a year is intercalated. Just as a month is added to only at its end, so, a year is added to only at its end. R. Yirmiyah says: Just as uncleanliness constrains (the offering of the Pesach [viz. Numbers 9:10] and (the advent of) spring constrains, then just as the (constraint in) the first is not less than thirty days, so, (that in) the second must be no less than thirty days. __ But perhaps (an intercalation is to be made backwards] even if spring arrives prematurely? It is, therefore, written (Exodus 13:10) "and you shall keep this statute (of Pesach) in its time" — You add to its time, but you do not subtract from it.

... This is the [first] month to you ~ The first ancestors did not count it. Is it possible that the first ancestors did not count it? Behold, the Torah was not given in their days, but from the moment Torah was given and onward, the first comers (ie, the other nations) would count. No, this is not possible: the text says - "for you". You count it [as the first month], and the other nations do not.[First of] Months - Is it possible [that this is the first month] for [beginning of] years and shemittah [cycles] and Jubilee [cycles]? [No,] the text says "first of months" - for months it is the first, but it is not the first for the [beginning of] years, nor shemittah [cycles], nor Jubilee [cycles]. "It is first for you [all]" - and not for the tenth part of animals, and not for the fruits. Why is it the first for kings and festivals and donations of shekels and documents of houses? "It is first" - because of the general principle that is said: "guard the month of Aviv". How do you guard it? Add days to it. Is it possible that, if it was a year that lacks ten or twenty days one would complete it like that [with just ten or twenty days]? [No,] the text says "month" - there is no [adding] less than a month. Is it possible that, if it was a year that lacks forty or fifty days one would complete it like that [with forty or fifty days]? [No,] the text says "month" there is no adding more than a month. Is it possible to complete the year with Nisan? [No,] the text says "it is the first month to you" - one Nissan you make, and you do not make two Nissan. Is it possible to complete the year with Tishrei? [No,] the text says "guard the month of Aviv" - guard the month that is close to Aviv [spring], and which one is it? Adar. Is it possible that if it is an early year [ie, autumn comes early] you would add days to it? And similarly if it was a late year you would take away days from it? [No,] the text says "and you will guard this rule in its appointed time from year to year" - a year is never less than twelve months. "It is the first month for you of the months of the year" - a second Nisan should not be done.