ISBE Easter:
es'-ter (pascha, from Aramaic paccha' and Hebrew pecach, the Passover festival):
The English word comes from the Anglo-Saxon Eastre or Estera, a Teutonic goddess to whom sacrifice was offered in April, so the name was transferred to the paschal feast.
The word does not properly occur in Scripture, although the King James Version has it in Ac 12:4 where it stands for Passover, as it is rightly rendered in the Revised Version (British and American). There is no trace of Easter celebration in the New Testament, though some would see an intimation of it in 1Co 5:7. The Jewish Christians in the early church continued to celebrate the Passover, regarding Christ as the true paschal lamb, and this naturally passed over into a commemoration of the death and resurrection of our Lord, or an Easter feast. This was preceded by a fast, which was considered by one party as ending at the hour of the crucifixion, i.e. at 3 o'clock on Friday, by another as continuing until the hour of the resurrection before dawn on Easter morning. Differences arose as to the time of the Easter celebration, the Jewish Christians naturally fixing it at the time of the Passover feast which was regulated by the paschal moon. According to this reckoning it began on the evening of the 14th day of the moon of the month of Nican without regard to the day of the week, while the Gentile Christians identified it with the first day of the week, i.e. the Sunday of the resurrection, irrespective of the day of the month. This latter practice finally prevailed in the church, and those who followed the other reckoning were stigmatized as heretics. But differences arose as to the proper Sunday for the Easter celebration which led to long and bitter controversies. The Council of Nice, 325 AD, decreed that it should be on Sunday, but did not fix the particular Sunday. It was left to the bishop of Alexandria to determine, since that city was regarded as the authority in astronomical matters and he was to communicate the result of his determination to the other bishops.
But this was not satisfactory, especially to the western churches, and a definite rule for the determination of Easter was needed. By some it was kept as early as March 21, and by others as late as April 25, and others followed dates between. The rule was finally adopted, in the 7th century, to celebrate Easter on the Sunday following the 14th day of the calendar moon which comes on, or after, the vernal equinox which was fixed for March 21. This is not always the astronomical moon, but near enough for practical purposes, and is determined without astronomical calculation by certain intricate rules adopted by ecclesiastical authority. These rules involve the Dominical Letters, or the first seven of the alphabet, representing the days of the week, A standing for the first day of the year and the one on which Sunday falls being called the Dominical for that year. There are also involved the Golden Numbers and the Epacts, the first being the numbers from 1 to 19, the cycle of the moon when its phases recur on the same days of the year, the first of the cycle being that in which the new moon falls on January 1. The Epacts indicate the moon's age at the beginning of each year. Easter was thus fixed by these rules, but another difficulty arose when the Gregorian calendar was adopted in 1582, the difference between it and the Julian being then 10 days. This of course affected the determination of Easter, and its celebration by the Greek church, which has never admitted the Gregorian calendar, occurs usually at a different time from that followed by the western churches. This difference may be as much as five weeks and it may occur as late as April 30, while in the West it cannot occur later than April 25 nor earlier than March 22. Occasionally the two come together but this is rare, since the difference between the two calendars is now 13 days.
The Easter feast has been and still is regarded as the greatest in the Christian church, since it commemorates the most important event in the life of its Founder.
Letter of Constantine Augustus to the [Roman] Churches
When the question arose concerning the most holy day of Easter, it was decreed by common consent to be expedient, that this festival should be celebrated on the same day by all, in every place. For what can be more beautiful, what more venerable and becoming, than that this festival, from which we receive the hope of immortality, should be suitably observed by all in one and the same order, and by a certain rule. And truly, in the first place, it seemed to everyone a most unworthy thing that we should follow the custom of the Jews in the celebration of this most holy solemnity, who, polluted having stained their hands with a nefarious crime, are justly blinded in their minds.
It is fit, therefore, that, rejecting the practice of this people, we should perpetuate to all future ages the celebration of this rite, in a more legitimate order, which we have kept from the first day of our Lord’s passion even to the present times. Let us then have nothing in common with the most hostile rabble of the Jews. We have received another method from the Savior. A more lawful and proper course is open to our most holy religion. In pursuing this course with a unanimous consent, let us withdraw ourselves, my much honored brethren, from that most odious fellowship.
What some call Easter is the Gregorian calendar based, lunar calculated Passover: first Sunday following the first full moon following the vernal equinox... instead of the Jewish reckoning. They do occasionally align 
