Exodus 12:2

2. This month (Abib or Nisan) shall be unto you the beginning ( head) of months… first month of the year Hitherto the year had commenced with the month Tisri, (or September,) but henceforth the year was to be reckoned from Abib, the month of Israel’s birth . Abib signifies “an ear of grain;” it was the month when barley ripened, corresponding with our close of March and beginning of April . The Hebrew months were lunar, and Abib was the month commencing with the new moon just after or just before the vernal equinox. This was the sacred year, by which the festivals were reckoned; but the civil or common year was still reckoned from Tisri. The passover was, then, instituted some time in the month of Israel’s deliverance, but not after the final interview with Pharaoh described in the last chapter, since four days, inclusive, were to elapse after the choice of the lamb before the passover.