How did Christians get from the biblical feast of Passover to Easter sunday? Can any one offer an educated answer of how the Christian Church moved from Passover on the 14th day of the first biblical month to the day we now call Easter?

(You can even pretend Passover and Easter are synonymous tittles!)

Making the question more clear:

The Bible clearly says,

Leviticus 23:5  In the fourteenth day of the first month at even is the LORD'S passover.

This is as well more elaborated in Exodus 12.

Jesus and the Apostles all would have used this date, see Luke 22, Mark 14 and Mathew 26.
Jesus disciples would have kept the Passover  according to the Jewish luni-solar and Agricultural based calendation of the day,  which calls the first month Abib, 

Deuteronomy 16: 1  Observe the month of Abib, and keep the passover unto the LORD thy God: for in the month of Abib the LORD thy God brought thee forth out of Egypt by night.

Today the celebration in West now called 'Easter' is not held according to the prior calendar law of the Lord but is kept on 'Easter Sunday'.

How do Catholics and Protestants do this with scriptural authority?

Make your answer clear from the Bible.  Show where in the Bible a prophet, Jesus or his disciples/apostles tell chrisians how to keep this feast we call Easter?

Or if you believe you do not need scriptural authority please tell why in the answer you give regarding the question:

How did Christians get from the biblical feast of Passover to Easter sunday?

(Please do not give any Ishtar speculations, this question does not concern paganism...thank you.)
Easter and Passover are two different celebrations. Passover celebrates the escape of the Israelites from Egypt and the sparing of them from the plague of killing the firstborn. Easter celebrates the much more significant resurrection of Jesus Christ from the dead. You will find these events described in all four of the Gospels in the New Testament, Matthew Mark Luke and John. The resurrection is foreshadowed by Passover and occurred at the same time of year. 

Passover is a strictly Jewish celebration and there is no thought that non-Jews would celebrate it. Easter is a celebration for all people that believe in Jesus, whether Jewish or not. Passover celebrations waned as the church became more and more non-Jewish, and an annual Paschal celebration of the Resurrection was well established by the second century AD.

I hope that helps.