"This day you go out in the month of Aviv" (Exodus 13:3) — a verse that seems to state the obvious. Of course Israel left in the month of Aviv (spring). The Torah already told us that. So why repeat it?
The Mekhilta answers: the repetition teaches that the year of the Exodus did not require intercalation. In the Jewish lunar calendar, a leap month (a second Adar) must periodically be added to keep the calendar aligned with the agricultural seasons. Without this correction, Passover would drift through the year, sometimes falling in winter, sometimes in summer. The intercalation ensures that Passover always falls in the spring — in the month of Aviv, the month of ripening grain.
But the year Israel left Egypt, no adjustment was needed. The lunar and solar calendars happened to be perfectly aligned. The month of Aviv fell exactly where it should — in the spring — without human intervention. The verse "this day you go out in the month of Aviv" is not redundant. It is confirming that the calendar was naturally correct that year.
The Mekhilta sees this as more than astronomical coincidence. God orchestrated the timing of the Exodus so precisely that even the heavens cooperated. The moon and the sun, the lunar months and the solar seasons, aligned perfectly for Israel's departure. No rabbinical court needed to add a month. No calendrical adjustment was required. The cosmos itself was synchronized with the plan of redemption. When God chose the moment to free His people, He arranged the very orbits of the heavenly bodies to match.