
The first Passover night (Nisan 14) preceded the Exodus from Egypt, the Red Sea crossing, the Cloud that led the people by day & the Fire at night...
In Exodus 12, G-d instructs the people to select a lamb on Nisan 10, tend to that lamb til Nisan 14 when they were to: slaughter it & place its blood on their doorposts so that the L-rd would pass over their homes; then roast the lambs and eat the meat with unleavened bread, sharing within families and within the community. The very next morning they were led out of Egypt.
So in the Hebrew mindset Passover is celebrated to commemorate the going out of Egypt and the great miracles which the L-rd used to free the people. All of this beautiful and beyond understanding.
But there is so much more: G-d instructed His people to offer daily sacrifices of various kinds and for various purposes and much of those sacrifices were animals and that is because according to Leviticus 17:11 - "For the life of the flesh is in the blood, and I have given it to you upon the altar to make atonement for your souls; for it is the blood that makes atonement for the soul."
Yeshua is THE sacrificial Lamb, only His Blood is pure, holy, able to make atonement for ALL. And so He had to be crucified on Passover. But He also wanted to gather with His friends to celebrate Passover, commonly known as the Lord's Supper or the Last Supper. And during that Passover seder, it was after He washed their feet, told them to eat His Flesh & drink His Blood, and the prayers in the garden, that He was arrested and then beaten & crucified on the same day that the Passover lambs were to be slaughtered for Passover. Not incidentally, G-d instructed the people that they had to eat the entire lamb, NONE of it could be left over. All of this preparing the people, looking forward to when Yeshua would say:
John 6:53-58 "Then Jesus said to them, 'Most assuredly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in you. Whoever eats My flesh and drinks My blood has eternal life, and I will raise him up at the last day. For My flesh is food indeed, and My blood is drink indeed. He who eats My flesh and drinks My blood abides in Me, and I in him. As the living Father sent Me, and I live because of the Father, so he who feeds on Me will live because of Me. This is the bread which came down from heaven—not as your fathers ate the manna, and are dead. He who eats this bread will live forever.'"
So that is why it's a beautiful thing to have a seder an evening earlier.
And that mark which those who celebrate Passover are marked with...is the Blood of the Messiah, Yeshua, which literally means God Saves.