Biblically we know that Christ died on the 14th of Nisan (Passover). Based on the Jewish day ending at sundown, add three days/night to determine the day He rose. Who cares what the Council of Nicea determined or any ecumenical council. What does the bible say? There was a Sabbath for the feast and THAT particular year, it fell on the weekly Sabbath. It's not always a Sunday since that's just something invented by the ecumenical councils.