Harking back to some 50 years or more to my time at St Bede's College, Manchester, when theology was able to be studied by those boarders who hoped to train for the priesthood, I can remember having certain dates in history of the Church impressed on my memory. I feel that at the Council of Nicea in 325AD, a decree was made that the start of Lent (Quaresima) would in future be celebrated on the first Sunday after the full moon of the Spring Equinox. Some in the Church followed the Julian calendar and others followed the Gregorian calendar.
In Britain, in 664AD, the Synod of Whitby was convened to decide which of the two calendars should be followed here.