14 September 1752: Britain adopts the Gregorian calendar and “loses” 11 days

If, in 1752, you'd arranged to do anything on any day between 2 September and today, you'd have had a hard time getting it done. And if it was your birthday on any of those days, you'd have a right to feel upset – no matter how big your cake or how much jelly you'd laid on, nobody would have come to your party. Because those days didn't exist – the British Calendar Act of 1751 had abolished them.

Until this day in 1752, Britain was using the Julian calendar, designed by Sosigenes of Alexandria and introduced by Julius Caesar in 46BC. Just like the current calendar, the Julian calendar had 365 days, with 366 days every fourth year. But that missed out a day every 128 years, which meant the year was effectively 11 minutes short, when compared with the solar year.

That was a big deal to some, and especially to the Pope. Before 1582, Easter was celebrated on the spring equinox, which everyone agreed would be 21 March. The trouble was, with every year being 11 minutes too short, Easter was hurtling towards Christmas at the rate of just over a week every thousand years. That, clearly, had to be fixed.

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In 1582, Pope Gregory XIII (or presumably, mathematically minded monks working for Pope Gregory) came up with a new calendar that would help regulate Easter more reliably. And so in that year, the Catholic countries of France, Italy, Portugal and Spain changed calendars, and lost ten days.

Britain, being a good non-Catholic country, thought it was a dangerous load of Papist nonsense, no matter how sensible it might actually have been. So we steadfastly stuck to the Protestant, but fundamentally wrong, way of doing things for another 200 years, by which time we were 11 days out of sync.

1752 was also the first year that began on 1 January. Until the Calendar (New Style) Act of 1750, 25 March was considered to be the start of the new year. So 1751 was just 282 days long, running from 25 March until 31 December.

The change to the Gregorian calendar is also the reason the new tax year starts on 6 April.