What did the Feast of Passover refer back to?
The first of Israel's Feast Days, Passover, referred back to the Passover Lamb in Egypt. Israel had just come out of Egypt when God was giving them the Law, the Tabernacle worship, and the Feast Days. This first feast pointed to the Passover Lamb in Egypt.
Les explains from Leviticus 23 that Israel was being instructed after coming out of Egypt, as God gave them the Law, the Tabernacle worship, and the Feast Days. The first Feast was Passover, and it referred back to the Passover Lamb in Egypt. He notes the Jew still celebrates these Feast Days today, perhaps under a somewhat different setting. The sequence then continues to the Feast of Unleavened Bread and the Feast of First Fruits.