In addition to missionaries from Kent the Celtic churches in Ireland began their own conversion programme, using Lindisfarne as a base. Soon, they too were having success amongst the English kingdoms in the north, even returning Northumbria to Christianity. The existence of both missions created problems. The Celtic church had been out of touch with Rome for centuries, and many of Rome's new theological ideas had passed them by. In particular, they used an older and different way of calculating the date on which Easter was to be held.
Easter is a moveable feast, the date of which is determined by the phases of the moon.
Leading up to Easter is Lent, a period of penance lasting forty days during which only one meal a day was allowed and flesh and fish were forbidden. Since the Celtic Church and the Roman Church had different dates for Easter, this led to the king of Northumbria and his court feasting because they used the old Irish date, while the queen was still in her Lenten fast in accordance with the new Roman date. There were other problems and something had to be done. At Whitby in 663 the two Churches met with Oswiu, king of Northumbria, and asked him to choose between them. Both sides put forward their case, bringing forward their best scholars.