The ten plagues on Egypt were over.  The tenth one, which was the death of all firstborns without lamb's blood brushed on the doorposts of their homes, was "the straw that broke the camel's back".  Pharaoh finally consented to let Israel, God's firstborn "son" (Exodus 4:22), depart from his land to service the Lord.  

Israel heard how they were to henceforth commemorate the original Passover experience with an annual Feast of Passover, as well as a Feast of Unleavened Bread (Exodus 13).  They were never to forget what the Lord did for them in delivering them from their bondage in Egypt!  It was ALL a prophetic picture of the One Who would literally BE the Passover Lamb and Who would literally BE unleavened (without sin) so He could save ALL firstborns who willingly applied His blood to "the doorposts" of their hearts (and, thus, become "second borns" - born again).  

Powerful lesson on PASSAGE and PRAISE!