Introduction
Until 1752, the British Isles used the Julian Calendar brought here by the Romans in the first century AD. It had its problems, as even vocal champion St Bede acknowledged; but when Rome updated it in 1582 they trampled needlessly on ancient Church rules, offending the Greeks and Russians, and the Reformation was in full swing, which meant the English were in no mood to comply either.
AT the close of the tenth century, peoples from the eastern borders of the Roman Empire to newly-Christian Russia and even Britain shared one calendar, the Julian, introduced by Julius Caesar in 45 BC. And thanks in no small degree to eighth-century Northumbrian monk Bede and his best-seller ‘On the Reckoning of Time’, they also shared one Easter, looking up the first full moon of Spring in tables devised in fourth-century Alexandria, and taking March 21st as a convenient first day of Spring.*
However, Bede himself noted that the Julian civil calendar had a flaw, and was increasingly out of step with the heavens. In his time, the discrepancy was three days; by the sixteenth century the drift had reached ten, and the tables for predicting the Paschal full moons required some tidying up too.* Eventually Pope Gregory XIII responded by rolling out a new calendar in 1582, the Gregorian, undoubtedly more accurate, but bitterly controversial, not least in England.
For the background, see our post Bede and the Paschal Controversy.
By 1752, when England introduced the modern Gregorian calendar, the difference was eleven days, and since 1900 it has increased to thirteen.