The First Passover:
The first Passover took place when the children of Israel were still in bondage in the land of Egypt. God's instructions were to take a male lamb without blemish and kill it. The children of Israel were to take the blood from this lamb and put it on the two side posts and the upper door post of the house where they were going to eat the lamb. The Egyptians would not let the children of Israel go so that night God would smite the Egyptians with the tenth and last plague. The tenth plague was that God would smite all the first born of man and beast in the land of Egypt. The reason for the blood on the door posts was that when God saw it, He would "passover" that house and not smite the first born. You can read about the first Passover in Exodus 12:1-28

What Passover is Today:
Passover for Christians today is different than it was in Old Testament times. The fact that God told the children of Israel to use a male lamb without blemish should have made you think of what we are told in the Gospel of John.
John 1:29
V29 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, Which taketh away the sin of the world.
Jesus Christ was the only person to ever walk this earth in a flesh body and not sin. Everyone else will sin at some point in their life. It was God's plan for Jesus Christ to shed His blood on the cross as He is the only perfect sacrifice for the forgiveness of sins.
Are you beginning to get the idea why God gets so upset when Christians celebrate easter and not Passover? Easter is a heathen tradition of men and not of God. Nowhere in the Bible will you find God telling His children to color eggs and hid them. If you don't believe me, get a Strong's Concordance and look up the word easter for yourself. You will see that the Greek word used means Passover.
I Corinthians 5:7
V7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
Jesus Christ has become our Passover. In the Old Testament, the shedding of animal blood was needed for the forgiveness of sins. Jesus Christ fulfilled that part of the law by shedding His blood on the cross. We need to believe in Him and repent of our sins.
Now that you know that Jesus Christ has become our Passover, you should understand why it makes God angry when Christians celebrate the heathen holiday easter instead of Passover. Passover should be a time to remember what God did for the children of Israel when they were still in bondage in the land of Egypt. It should also be a time to remember the price that Jesus Christ paid on the cross. Giving His life in the flesh so we with faith and repentance can have forgiveness of our sins.