Before we go on, it is by no means a biblical requirement to practice these feast, since they find their fulfillment in Christ. I use these feasts and the celebration of them for teaching tools for my kids; and they have been quite effective. So, while we are under no obligation to practice them, I do think that when done correctly, they can be very useful in bringing the gospel to life.
In looking at how these feast unfold, Passover and the Feast of Unleavened Bread are held one right after the other. The Passover lamb was slain on the fourteenth of Nissan and the Feast of Unleavened Bread began the day after (15 Nissan). Actually, Numbers 28:16-17 may give reason as to why Jesus began his Passover seder on the fourteenth. Because these holidays are so close together, they are usually treated as one festival. In fact, Exodus 13:3-8 seems to teach that these are two names for the same festival.
The first three spring holidays overlap with each other:
1. Passover (Pesach) Nissan 9-14
2. Unleavened Bread (Hag HaMatzah) Nissan 15-16
3. Day of First Fruits Nissan 17