Sforno on Deuteronomy 16:1/ספורנו על דברים ט״ז:א׳

1שמור 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.

2ועשית פסח, on the first evening of the festival of matzot

3כי 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.

4לילה, 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.