Controversy over the Date of Easter
This article is one in a series on the history of the Church - you are currently on the topic that is in bold:
The First Council of Nicaea in 325 A.D. repudiated Quartodecimanism (Easter on the 14th of Nissan), and acknowledge exceptional authority of the patriarchs of the Churches in Alexandria and Rome over their respective regions.
So what was the big deal about the date of Easter?
Gentile vs. Jewish Christians
The Last Supper happened at Passover, which is the 14th of Nissan according to the Jewish Calendar. Jewish Christians began celebrating the Lord's Supper on this date. During the early years of the church, Jews were exiled from the city of Rome. As a result, Rome became the first all-Gentile Church for a time. Gentile Christians did not follow the Jewish Calendar, and decided to celebrate Passover on the first Sunday after the full moon after the March equinox.
Sixtus I was the sixth Bishop of Rome. In 117 A.D., he declared that any Bishop visiting Rome should not be accepted back without a letter of recommendation. This is the first time a Bishop tried exerting influence over another Bishop, and he was largely ignored by other churches at that time. Sixtus' doctrine is in contrast to the deeds of the third Bishop of Rome, Clement, who wrote the following to the Corinthians:
Polycarp, Anicetus and Easter
The Bishops in Rome continued with this feeling of supremacy until Polycarp (the disciple of John), became frustrated by their attempts to influence other churches over trivial matters and traveled to Rome to deal with the issue of the date of the Passover/Eucharist.
Anicetus (the 10th Bishop of Rome) submitted to Polycarp's rebuke, and an agreement was reached around 160 A.D. that each church should have the right to determine the date of the Passover/Eucharist independently.
Polycarp also influenced Anicetus to condemn certain heresies with more vigor.
Irenaeus, Victor and Easter
Victor was the 13th Bishop in Rome. He excommunicated all of the churches in Asia Minor for disagreeing with his interpretation regarding the date of the celebration of Easter. Most of the churches in Asia Minor had significant Jewish populations, while Rome had little Jewish influence. Irenaeus addressed Victor in a letter (only a fragment of which remains), warning him that if he persisted in the course on which he had entered, the effect would be to rend the Catholic Church in pieces. In 190 or 191, Irenaeus travelled to Rome to meet with Victor, who received and accepted the rebukes of Irenaeus. The debate of the date of the passover continued to be handled independently by each Church until the Council of Nicea.
The Council of Nicea and Easter
In 325, at the Council of Nicea, the first canon (unchanging law of discipline) was issued giving the patriarchs of Alexandria and Rome exceptional authority over other churches in their regions. It was also decided at this council that the Christian Passover must not be celebrated with the Jewish Passover, which was the custom of the disciples. The Roman Emperor Constantine enforced this doctrine by the physical suppression of forms of worship he considered unorthodox. 
Not eager to reignite the controversy of Easter, many historians have tried to spin the resolution of this controversy in a positive light. For example, the introductory note to Irenaeus' book "Against Heresies" includes the following note by the translators:
But just because something is called a "happy result" does not mean it is a "happy result". In fact, the "result" was the start of oppression by the Church of Rome over all other Churches around the world.
Columba and Easter
Ireland was outside of the Roman Empire, and Christianity developed independently in this region without Rome's influence. The Irish, like the Jews, had a calendar that did not agree with the Roman calendar, and celebrated Passover on a different day of the year.
The life story of Columba written in the 600's tells of prophecies concerning the date of Easter:
Easter Sacks The Influence Of The Irish Monks
After Columba's passing, Abbot Segene of Iona sent Aedan as a missionary to evangelize Northumbria (England), who disputed with Pope Severinus in 638 over the date of Easter. Aedan converted the English simply by walking from village to village, politely conversing, and slowly winning their hearts to Christ, and established a Monastary at Lindisfarne. Aedan was succeded by Finan and then Colman. Colman was eventually evicted by the Christian Northumbrians after they accepted the Roman date of Easter (which the Ionian monks strongly protested), and he returned to Iona and later established a monastary on the island of Inishbofin off the west coast of Ireland, which remained until the 10th Century.
Conomail of Iona became very involved with the Easter controversy, and was finally replaced under questionable circumstances by Dunchad. Dunchad quickly adopted the Roman date of Easter, and established ties with Rome. King Nechtan IV of the Picts then expelled all of the Ionian monks as he wished to remain free from both Rome and Northumbrian influence.
The politics of the Roman Church brought about the fall of Iona, which was readily apparent to the Pictish kings. Lindisfarne was sacked by the Vikings in 793, and Iona in 795.