John Gill Commentary Exodus 34:18

"The feast of unleavened bread shalt thou keep. Seven days thou shalt eat unleavened bread, as I commanded thee, at the time appointed in the month Abib; for in the month Abib thou camest out from Egypt." — Exodus 34:18 (ASV)

The feast of unleavened bread shall you keep
Which was instituted at the time of their coming out of Egypt, and on that account, and then observed, (Exodus 12:15Exodus 12:18Exodus 12:19) (Exodus 13:6Exodus 13:7) and afterwards repeated, and the month expressed in which they were to keep it, and the reason of it, as it here follows, (See Gill on Exodus 13:15).