Deutéronome 16:1

Be mindful to keep the times of the festivals, with the intercalations of the year, and to observe the rotation thereof: in the month of Abiba to perform the pascha before the Lord your God, because in the month of Abiba the Lord your God brought you out of Mizraim; you shall eat it therefore by night.

the month of Spring is so called because it was during the Springtime that they left Egypt. The Dissenters have claimed that one may not perform the Paschal sacrifice until Spring-time. I have replied to their nonsense elsewhere [comment on Exodus 12: 2] (nevertheless, we must acknowledge that the court did examine whether or not Spring had arrived). Know that in the Hebrew language the months have no proper names. The names that appear in Scripture — like Nisan [Esther 3: 7], Sivan [Esther 8: 9], Kislev [Nehemiah 1: 1], Ṭevet [Esther 2: 16], and Adar [Esther 3: 7] — are all Aramaic. Consequently, Scripture had to tell us, “…this being the month of Nisan” [Esther 3: 7]. The only Hebrew terms that we find are the descriptive terms “Splendor” [I Kings 6: 1], “Mighty” [I Kings 8: 2], and “Wilt” [I Kings 6: 38]. The original rule was that the first month (the month from which the others are counted) was to be reckoned with the coming of the Spring — whether the previous year had twelve or thirteen months being of no concern. However, our ancestors, of blessed memory, added other considerations, in addition to the Spring [Sanhedrin 11b]; and all of their words are true.

at night God brought you out from under the authority of Pharaoh, and they left shortly afterward, as I have explained at greater length [comment on Exodus 12: 31].

KEEP THE MONTH OF AVIV. The commandment of the festivals [mentioned here] is explanatory, for He has already mentioned it. Now, in the Book of Leviticus He mentioned with reference to the festivals, And ye shall bring an offering made by fire unto the Eternal, but here he did not mention them [the offerings] at all. Instead he commanded Israel to ascend on their account [to celebrate the festivals] to the place which He will choose, and rejoice before Him. For just as he commanded that one bring the Second Tithe before G-d, and also the firstling,, 15:19. and eat them there in the place that G-d will choose, he added [here] to make it clear that they will all be further obligated to come before G-d and celebrate before Him on three festivals and rejoice with the peace-offerings that they will offer before Him. Now he did not mention the times of the Festivals of Unleavened Bread and Tabernacles on which dates they occur, but he briefly mentioned their months, for He had already stated everything there.

שמור את חודש האביב, make sure that the month of Nissan always occurs in the spring, by manipulating the calendar, if necessary and inserting an extra month of Adar. By doing this the solar orbit (year) and the lunar orbits, (12 moons equaling a year) can be brought into line with each other.

ועשית פסח, on the first evening of the festival of matzot

כי בחודש האביב הוציאך, because it was G’d’s will that your departure from Egypt should be at the time when the month signaling the beginning of spring should occur, astrologically a time when the zodiac sign of the lamb ascends in tandem with the sun, and is opposite the moon, the latter symbolising the Jewish people, as opposed to the Egyptians who worship the sun as a major deity.

לילה, on that occasion, the sun/lamb being opposite the moon occurred at night. Seeing that one cannot offer a sacrifice at night, the Torah arranged for the Passover to be offered on the afternoon preceding that night. As a commemoration of that event, it was decided not to change the time of offering the Passover even though the astrological/astronomical data change from year to year.