Leviticus 23 challenges Israel to rehearse God given holidays or festivals as a way of proclaiming Him to the rest of the world. That rehearsal can look back, forward or both. The first festival was the Sabbath or Shabbat, a weekly reminder that God created in six days and rested the seventh and so should we. The remaining seven festivals are annual and the first was Passover or Pesach.
Pesach was a reminder for Israel to look back and remember how God "passed over" Jewish homes, marked with the blood of a lamb, as he brought judgment on Egypt. It was also a rehearsal for what was to come, a passover lamb, Jesus, that came to die once for all.
The second annual festival that God ordained in Leviticus 23:6 was the Feast of Unleavened Bread or HagHaMatzot. The Feast of Unleavened Bread began on the 15th day of Nisan, just 24 hours after Passover. It is a reminder that there was no time to allow bread dough to rise as they escaped Egypt after the angel of death destroyed Egypt's first born.
The leaven that was left out of the bread was a picture of sin, just as a little leaven will cause a lump of dough to become puffed up, so to just a little sin causes us to become puffed up and prideful in our own eyes. So the leaven (sin) was purged from the home and the bread they took with them was flat.
A reminder for the escaping Jews that God directed that they be sustained with a bread without leaven AND a promise that one day he would provide a sinless sacrifice, Jesus, whose body would be broken just as the flat bread was broken to feed the entire family.