Short answer:
Good Friday and Passover occur on different days because they are not the same thing.
Passover is the 14th day after the first new moon following the spring equinox, which can occur on any day of the week.
Good Friday is the Friday before the first Sunday that occurs after Passover.
Longer answer:
The Nicene Council did not make its decisions because of any “anti-Semitism.”
The churches from the very beginning were gathering together on Sundays and partaking of the Christian Passover, which is the Eucharist, the Lord’s Supper, when we partake of the bread and wine which symbolize the Lord's body and blood: "For even Christ our Passover is sacrificed for us." (1 Corinthians 5:7) And that’s why Christian worship is on Sunday, not because Christians changed the Sabbath from Saturday to Sunday, in Christianity we find our rest in Jesus, not in a day of the week, and certainly not in the Law. But we Christians come together to partake of our Passover on the day on which our Lord rose from the dead, Sunday, the 8th day, the beginning of the New Creation.
However, in the earliest years of the faith, the churches of Syria and Mesopotamia (Babylon) were celebrating Easter on the 14th of Nisan according to Jewish reckoning regardless of whether that day fell on Sunday or not, which was called the “quartodeciman” view, while the rest of Christendom was celebrating Easter on the Sunday following Passover.
It wasn’t an issue until a man named Melito wrote a book teaching the quartodeciman view and then it became a controversy. That was one of the two principal reasons the Council of Nicea was convened, because “it was considered scandalous that while some churches were engaged in fasting and penitence, others should be indulging in festivities and relaxation.” (Eusebius, Council of Nice, Appendix to Ecclesiastical History)
The problem is that Passover and Easter are not the same thing.
The Christian Passover is the Eucharist, the Lord’s Supper, which most churches partake of on a weekly basis and have done so since the time of the Apostles which established the practice during New Testament times.
Easter, however, is the celebration of the resurrection, which did not occur on Passover, but on the Sunday following Passover.
Thus the unanimous decision of Christendom was that Easter (the celebration of the Lord's resurrection) should be observed on the first Sunday following Passover as that was the day on which our Lord rose from the dead.
The comment that the Christian Church should not be celebrating the Lord’s Resurrection "with the Jews" (according to the custom of the Jews ... ergo, on the 14th Nisan) is not anti-Semitic, it’s anti-legalism. Christians don’t keep the feasts according to the Jewish manner, by the old letter of the Law, but we keep the feasts according to the spirit of the feasts which was fulfilled by Jesus.
And the reason the church adopted a new calendar was because they were still using the variable Jewish calendar which was not a fixed calendar but was established by visual observations in Jerusalem of when the new moon of the spring occurred but also whether or not the barley harvest was mature enough. This made it difficult for Christians, who were spread far and wide, to know when Easter was and therefore to be able to celebrate at the same time. So the Churches of Christendom elected to adopt the Metonic Cycle calendar which was developed in Egypt which is based on a 19-year lunar cycle which brings the lunar calendar into sync with the solar calendar every 19 years. This allowed the Churches to determine ahead of time when Easter would occur so they could all celebrate together.
This change from using the variable Jewish calendar to a fixed calendar certainly wasn't anti-Semitic as the Jewish scholar Hillel II also adopted the same Metonic 19-year cycle for the fixed Jewish calendar in 670 A.D.
In Christ,
Pilgrimer
Since the subject of the chronology of the Passion Week has come up I'd like to post an outline of the week that is based on Scriptural and historical sources. I'm afraid much of the arguments on here are not historically accurate as to what the Law commanded in respect to the observance of Passover or how it was historically observed by the Jews at the time of Christ. The following is based on taking all that information into account.
Easter Week
Paschal Week Chronology
6 days before Passover
Friday – Nisan 8 – late afternoon - Jesus arrives in Bethany from Jericho
Friday – Nisan 9 – sunset – Sabbath supper at home of Simon
5 days before Passover
Saturday – Nisan 9 – Sabbath - Jesus rests in Bethany; multitudes come from all over to see Jesus and Lazarus; chief priests and Pharisees plot to arrest both Jesus and Lazarus
4 days before Passover
Sunday – Nisan 10 – Palm Sunday, Jesus enters Jerusalem; lambs selected for Passover; Jesus cleanses Temple for the 2nd time
3 days before Passover
Monday – Nisan 11 – Jesus teaches the multitudes
2 days before Passover
Tuesday – Nisan 12 – Jesus’ denunciation of Jerusalem; Mt. Olivet discourse
1 day before Passover
Wednesday – Nisan 13 - no record in Gospels how Jesus spent the day; possibly in Bethany and last night spent there
Wednesday – Nisan 14 – at sunset Jews begin to search their homes with candles for leaven
Passover
Thursday – Nisan 14 – morning – no leaven eaten after 10:00 a.m.
Thursday – Nisan 14 –noon– leaven cemonially destroyed by burning or by dispersing to the winds
Thursday – Nisan 14 – afternoon – evening sacrifice moved up from 3:30 p.m. to 1:30 p.m. and offered at 2:00. Passover lambs sacrificed from 2:00 p.m. to 5:00 p.m. Seder prepared
1st Day of Unleavened Bread
Thursday – Nisan 15 – evening – Last Supper/Seder eaten (roasted Passover lamb and 1st Passover Chagigah eaten {breast and shoulder of voluntary thank-offering})
Thursday – Nisan 15 – night – Jesus arrested, brought before priests and elders
Friday – Nisan 15 – morning – Jesus brought before Pilate; Pharisees refuse to enter judgment hall as defilement would prevent them from eating the obligatory 2nd Passover Chagigah
Friday – Nisan 15 – 9:00 in the morning – Jesus crucified
Friday – Nisan 15 – 12:00 noon– darkness
Friday – Nisan 15 – 3:00 in the afternoon – Jesus dies
Friday – Nisan 15 – late afternoon – Jesus' body placed in tomb
Friday - Nisan 15 - before sunset women purchase and prepare spices for Jesus' burial (festival sabbaths allowed certain work necessary for preparation for the feasts, shops were allowed to be open to provide pilgrims with necessary items to keep the feast); High Sabbath preparations made
Sabbath (a "High Day")
Friday – Nisan 16 – sunset – High Sabbath begins (2nd feast at which mandatory 2nd Passover Chagigah is eaten)
Friday – Nisan 16 – night – women rest
Saturday – Nisan 16 – Sabbath – women rest
Saturday - Nisan 17 - night - Rabbinic Law requires that work not be resumed during night following Shabbat
1st Day of the Week
Sunday – Nisan 17 – morning early – women come to tomb, Jesus is risen
Sources: The Gospels, Talmudic Tractate "Pesachim," Josephus "Antiquities."