Early Christians had probably originally celebrated Easter concurrent with the Jewish Passover (see Passover, Christian holiday), which was held on the fourteenth day of the first lunar month of the Jewish year, called Nisan, the day of the crucifixion according to John .
However, the First Council of Nicaea in 325 decreed that Christians should no longer use the Jewish calendar but universally adopt the practice of celebrating it on a Sunday, the day of the resurrection, as had come to be the custom in Rome and Alexandria.
The synod was held at a place called Streanæshalch, at a monastery of Hilda, herself a powerful Northumbrian noble and adherent to the Ionan Easter.
The identification of the location with the place later called Whitby is generally accepted, but not absolutely certain. The Ionan position was advocated by Colmán, Bishop of Northumbria.
The "Ionan" practice was that of the Irish monks who resided in a monastery on the isle of Iona (a tradition within "Celtic Christianity"), whereas the "Roman" tradition kept observances according to the customs of Rome.
In the kingdom of Northumbria, these two traditions coexisted, and each had been encouraged by different royal houses.
Nonetheless, the disunity did not result in problems as long as the well-respected Aidan was alive.Both accounts basically agree, though Bede gives a much lengthier discourse on the debate.The description of the proceedings, where King Oswiu presides and rules but does not engage in the ecclesiastics' debate himself, parallels examples of other synods in other sources, such as one in the Vita Sancti Bonifati by Willibald (where King Ine of Wessex performed the same function as Oswiu).Political and ecclesiastical concerns over the authority to change longstanding traditions, however, could cloud the issue: the Welsh and French both resisted the change for another century.The proper date of the celebration of the most significant Christian feast had already resulted in visible disunity in the Northumbrian court: Queen Eanfled of Bernicia and her court observed Easter on a different day than did King Oswiu.