Linkages between the equinox, Pagan celebrations & Easter:

Many, perhaps most, Pagan religions in the ancient Mediterranean region had a major seasonal day of religious celebration at, or following, the spring equinox. In one religion, Cybele, the Phrygian fertility goddess, had a consort who was believed to have been born via a virgin birth. He was Attis, who was said to have died and been resurrected each year during the period MAR-22 to MAR-25; i.e. at the time of the vernal equinox in the Julian calendar. 

Wherever Christian worship of Jesus and Pagan worship of Attis were active in the same geographical area in ancient times, Christians "used to celebrate the death and resurrection of Jesus on the same date; and pagans and Christians used to quarrel bitterly about which of their gods was the true prototype and which the imitation." Since the worship of Cybele was brought to Rome in 204 BCE, about 250 years before Christianity, it is obvious that if any copying occurred, it was the Christians that copied the traditions of the Pagans.

Today,  no consensus exists on the linkage between the Attis legend (and the stories associated with many other god-men) and Jesus Christ:

	Some religious historians believe that the god-man's death and resurrection legends were first associated with Pagan deities many centuries before the birth of Jesus. They were simply grafted onto stories of Jesus' life in order to make Christian theology more acceptable to Pagans in the Roman Empire.
	Ancient Christians had an alternative explanation; they claimed that Satan had created counterfeit Pagan deities with many of the same life experiences as Jesus had. Satan and his demons had done this, in advance of the coming of Christ, in order to confuse humanity.
	Most modern-day Christians regard the Attis legend as being a Pagan myth of little value. They regard Jesus' death and resurrection account as being an exact description of real events, and unrelated to the earlier Pagan traditions.

Among the Roman Catholic church and Protestant denominations, Easter Sunday falls on the first Sunday after the first full moon after MAR-20, the nominal date of the Spring Equinox. Its ancient linkages to sun and moon worship are obvious. Many sources incorrectly state that the starting date of the calculation is the actual day of the Equinox rather than the nominal date of MAR-20. Other sources use an incorrect reference date of MAR-21.

Easter Sunday can fall on any date from March 22 to April 25th. The year-to-year sequence is so complicated that it takes 5.7 million years to repeat. Eastern Orthodox churches sometimes celebrate Easter on the same day as the Roman Catholics and Protestants. However if that date does not follow Passover, then the Orthodox churches delay their Easter - sometimes by over a month.

http://www.religioustolerance.org/spequi2.htm