The seven-day Feast of Unleavened Bread is connected to Passover, since its observance began the day after Passover, Nisan 15th. During the seven days of the feast only unleavened bread could be eaten. While Passover commemorated the night of the deliverance from Egypt, the Feast of Unleavened Bread served to remind devout Jews of the circumstances of the Exodus and symbolized for them God’s call to holy living (Ex 12:39; Deut 16:3; Lev 23:6-8).
Concurrent with the remembrance of Passover— and in Jewish tradition witnessed in the New Testament and today often just called by the general season “Passover”—is the Festival of Unleavened Bread: “Then on the fifteenth day of the same month there is the Feast of Unleavened Bread to the LORD; for seven days you shall eat unleavened bread. On the first day you shall have a holy convocation; you shall not do any laborious work. But for seven days you shall present an offering by fire to the LORD. On the seventh day is a holy convocation; you shall not do any laborious work” (Leviticus 23:6-8).
The best place to pick up the thread is with the first of the holydays in the year, the Passover. There is a passage in one of Paul’s letters that long ago should have caused us to rethink this question. It was written to a Gentile church, and commentators tell us it was written during the Passover season. Paul is dealing with a serious problem in the church and in the process, he makes a connection to the Passover: "Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us. Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth." (1 Corinthians 5:7-8)