Exodus 12

"Then they shall eat the flesh on that night; roasted in fire, with unleavened bread and with bitter herbs they shall eat it... And thus you shall eat it: with a belt on your waist, your sandals on your feet, and your staff in your hand. So you shall eat it in haste. It is the Lord’s Passover." vv.8, 11

Read chapter 12
The passover meal wasn't some big nice Thanksgiving meal but more of a bitter meal. To have bread without leven is like eating cardboard and bitter herbs sounds like something that's gone bad and fermented (although I love fermented foods and have loved this). Notice it's even described how they're to eat the meal by wearing their belt and shoes with with a staff in their hands. This reason being that they were to eat it fast and to be ready. Way unlike a Thanksgiving meal today that is slowly throughout the whole day while wearing sweatpants and sitting around vegging for a few days after even. This is the Lord's Passover meal and the same meal celebrated in the New Testament when Jesus pastas with His disciples, typically called the Lord's Supper. That wasn't a new tradition started but a tradition to be kept in remembrance of Israel's delivery out of Egypt by the Lord. Jesus is the One who ultimately is the Passover lamb who save us from everlasting death.