The Big Split - the Passover/Easter Controversy
By the early fourth century the split between the church and the synagogue had pretty well solidified. But there were still churches, primarily in the province of Asia, who still celebrated Passover at the biblically prescribed time, the fourteenth of the lunar month of Nissan. Many of the other churches celebrated the week of Passover as Holy Week, but focussed their attention on the first day of the week, the day the churches had chosen to emphasize as a way of distancing themselves from Judaism. Other churches, some of them among the most influential, established a fixed Sunday according to the Roman calendar that would be Easter, in total independence from the traditions of the despised Jews.
Constantine, the emperor who embraced Christianity, wanted to establish uniformity in faith and practice throughout the empire. He convened the Council of Nicaea in 325 to decide on this and other issues. The conclusion, with some pressure from Constantine, was that it was insulting to Christianity to base its holidays on Jewish festivals, so the date of Easter was to be set, always on a Sunday, based on a fixed calculation from the Roman calendar. Thus the church continued to be weaned from its biblical foundation.