The Unleavened One
Leaven represents sin. Leaven causes things to "puff up" and become "prideful". Scripture refers to One who is without sin (Hebrews 4:15) who was sent as an offering for sin (Romans 8:3)... the Messiah Yeshua. Just as Messiah Yeshua was sinless we are called to be perfectly sinless (i.e. without leaven) for seven days... seven represents perfection or completion.
Paul wrote in 1 Corinthians 15:20 that Yeshua was resurrected as the first fruits from those who are "asleep" (i.e. dead). First fruit grain offerings were consumed completely and were to be offered without any leaven (Leviticus 2:11).
Consider a piece of matzah (unleavened bread) and how it pictures the Messiah:
Matzah is not puffed up with leaven. Messiah was without sin (Hebrew 4:15).
Matzah is pierced. Messiah was pierced for our transgressions (Isaiah 53:5).
Matzah is striped. Messiah was whipped and striped and by His stripes we are healed (Isaiah 53:5 KJV).
Traditionally speaking, the seventh day of Chag Hamatzot, the last day of the moed, is when the Israelites passed through the Red Sea. The entire nation was immersed and "baptized" so to speak. When we are spiritually baptized into Messiah we are baptized into His death (Romans 6:3) that we might live to righteousness (1 Peter 2:24).
Our next stop... some traditions of Chag HaMatzot.