Sforno on Leviticus 23:5/ספורנו על ויקרא כ״ג:ה׳

1'בין הערבים פסח לה; although the day on which the Passover is being slaughtered is the 14th of Nissan, not one of the days listed as מקרא קודש, the Torah relates to it as if it were, seeing that this sacrifice, signifying the Exodus from Egypt, the redemption of the Jewish people from slavery, is at the root of the whole legislation governing the Jewish calendar. The sages are empowered to see to it that the annual calendar date which the Torah legislated for this sacrifice to be offered coincides with the month in which the spring equinox occurs, so as to ensure that we do not violate the concept of היום אתם יוצאים בחודש האביב, “on this day you left (Egypt) it was during the month of spring.” The concept of “spring” and its message of resurrection of nature which had hibernated during the winter is so strong that it is part of the אלה מועדי ה' concept. (compare Deuteronomy 16,1)