Wikipedia
The term "Quartodecimanism" (from the Vulgate Latin quarta decima in Leviticus 23:5, meaning fourteenth) refers to the custom of early Christians celebrating Passover beginning with the eve of the 14th day of Nisan (or Aviv in the Hebrew Bible calendar), which at dusk is biblically the "Lord's passover".
The modern Jewish Passover and Feast of Unleavened Bread is seven days, starting with the sunset at the beginning of Nisan 15. Judaism reckons the beginning of each day at sunset, not at midnight as is common in Western reckoning. The biblical law regarding Passover is said to be a "perpetual ordinance" , to some degree also applicable to proselytes , but what it means to observe biblical law in Christianity is disputed.
Regarding the chronology of Jesus, some claim the Gospel of John (e.g., , , ) implies that Nisan 14 was the day that Jesus was crucified in Jerusalem and that the Synoptic Gospels instead place the execution on the first day of Unleavened Bread . In Ancient Israel the first day of Unleavened Bread, a special Holy Sabbath, was on Nisan 15 and began a seven-day feast to the Lord . By the time of Christ, many customs in regard to the festival had changed, notable among them the intermixing of the two festivals in some customs and terminology. The eight days, passover and the feast of unleavened bread, were often collectively referred to as the Passover, or the Pesach Festival.