Northumbria in 664 had two Christian traditions in one kingdom. The north had been converted from Iona, in the Irish tradition, and used an older method of calculating Easter and a different form of the monastic tonsure. The south looked to Canterbury and the Roman practice. King Oswiu followed the Ionan reckoning and his queen the Roman, with the result that in some years the court celebrated Easter while the queen's household was still keeping Lent.
The synod met at the double monastery of Streanaeshalch, ruled by the abbess Hilda, who belonged to the Ionan tradition and hosted the assembly that decided against it. Colman of Lindisfarne argued for the custom of Columba; Wilfrid argued for the practice of Rome and the wider church. The king, by Bede's account, asked whether both sides agreed that Peter held the keys of the kingdom, and on being told they did, ruled for the tradition that claimed Peter's authority.
The immediate matter was calendrical, but the decision aligned the Northumbrian church with Rome and with the Latin West rather than with the Irish mission that had converted it. Colman resigned his see and withdrew with those who would not conform, first to Iona and then to Ireland. The Ionan reckoning persisted elsewhere for decades and Iona itself conformed in the eighth century.