Exodus 12:18
In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even. In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even.
Exodus 12:18 belongs to the solemn ordinance of the Passover instituted on the night of the last plague upon Egypt, which was more severe than any plague before it. That final judgment came after the king and his idolatrous priests opposed to the last the request to let Israel go. The Egyptian people themselves wanted Israel to leave. Moses made known beforehand to Pharaoh and to the people of Egypt as well as to the Israelites the nature and effect of that last plague. That night was terrible to the Egyptians and glorious to the people of God.
Before striking all the firstborn of Egypt at midnight, God gave direction through Moses for Israel's departure and preservation from the coming judgment. Each family, alone or joined with others, was to eat unleavened bread with roasted lamb flesh and bitter herbs that night, with loins girded, shoes on feet and staff in hand, eating in haste because it was the Lord's passover. The blood of a lamb without blemish, sprinkled on the two side-posts and upper door-post, marked the house for the destroying angel to pass over. The eating of unleavened bread was part of the Passover supper eaten in ancient Egypt on the night of deliverance, gathered around the table in readiness for departure, joined to marking the doorway with blood, the passing of the destroying angel across the sky, and the death of the Egyptian firstborn with mourning in Egyptian homes. The law of the Passover expressly enjoined eating unleavened bread and keeping all leaven out of the houses during the feast, a requirement that was strictly observed.
The unleavened bread eaten with the paschal meal was appointed to keep Israel's deliverance from Egypt in memory, to make that deliverance vivid and lasting in the hearts of those who took part, and to turn the people's minds from worldly cares to God's works, mercy, and faithfulness. The Passover lamb pointed to the death of Christ, and the blood on the doorposts symbolized the blood of Christ cleansing believing sinners and sheltering them from judgment, a provision for salvation to be acted on in faith. Putting away leaven means that all who would receive life and nourishment from Christ must put away the leaven of sin. The account of God delivering Israel from Egypt connects that deliverance with God bringing Israel to Sinai to hear His law.