664. Synod of Whitby
Bede (at HE, iii.25, giving the date 664 at iii.26) and Stephen of Ripon (a much briefer note at Life of Bishop Wilfrid, chapter 10) record an assembly at Whitby to settle the question of the proper method of calculating the date of Easter. Present were Hild, abbess of Whitby, king Oswiu and his son Alhfrith, Colman, bishop of Lindisfarne, with his Irish clergy, and Agilbert, former bishop of the West Saxons, with the priests Agatho and Wilfrid. For a concise discussion of the debate, see W. M. Stevens's article in the Blackwell Encyclopedia of Anglo-Saxon England. In a nutshell, Wilfrid championed the Roman method of calculation while Colman championed the Irish method. King Oswiu declared in favour of the Roman method and so it was adopted as the standard, and Colman and his followers went back to Iona.
K. Harrison, The Framework of Anglo-Saxon History to AD 900 (Cambridge: 1976)