The Passover (Leviticus 23:5).
“In the first month, on the fourteenth day of the month at even, is Yahweh's passover.”
The first feast was the Passover which occurred on the fourteenth of Abib/Nisan (March/April), fourteen days after the new moon which marked the beginning of the new year as established in Egypt (Exodus 12:2). This was in remembrance of their deliverance from Egypt when Yahweh ‘passed over' their houses when he smote the firstborn of Egypt (Exodus 12:2-14; Exodus 12:21-36). Later the feast and the feast of unleavened bread would be seen as united together in one as ‘the Passover' (Luke 22:1). Whatever happened in their future Israel never forgot how God had delivered them from Egypt.
At this feast over a thousand years later (John 19:14-18) God's great Passover Lamb, the Lord Jesus Christ would be offered as the Lamb of God Who takes away the sin of the world (John 1:29; 1 Corinthians 5:7).