Skip to content
RandomUnder500
Entry No.003History2 min read · 450 words

The Year Ten Days Disappeared: How the Calendar Was Fixed

In October 1582, millions of people went to bed on the 4th and woke up on the 15th. Here is why the calendar had to skip ahead, and what it still affects.

A brass armillary sphere and a brass sundial on a dark wooden desk in warm afternoon light, with an hourglass and a leather-bound book behind them.
Fig. 1 — Measuring the year was never simpleAI-generated illustration

On Thursday 4 October 1582, people in Italy, Spain, Portugal and Poland went to sleep. The next morning was Friday 15 October. Ten days had been deleted from the calendar, on purpose.

A small error that added up

The calendar Europe had used for more than 1,600 years was introduced by Julius Caesar in 45 BC. It gave every year 365 days, plus a leap day every fourth year: an average of 365.25 days.

The trouble is that the solar year, the time it takes for the seasons to come round again, is about 365.2422 days. Caesar’s calendar ran roughly 11 minutes too long every year. That is about one day every 128 years.

By the 1500s the gap had grown to around ten days. The spring equinox, which the Church had fixed at 21 March for calculating Easter, was actually arriving around 11 March.

The fix

Pope Gregory XIII issued a decree in February 1582, based largely on the work of the Italian physician Aloysius Lilius. It did two things:

  • Removed ten days to bring the equinox back to 21 March.
  • Changed the leap year rule. Century years are leap years only if they divide by 400. So 1700, 1800 and 1900 were not leap years, but 2000 was.

That small change brings the average year to 365.2425 days. The Gregorian calendar now drifts by only about one day every 3,000 years. The days of the week were never interrupted: Thursday was still followed by Friday.

Not everyone switched at once

Catholic countries adopted the change straight away. Many Protestant and Orthodox countries refused at first, partly because it came from the Pope.

Britain and its colonies waited until 1752. By then the gap was eleven days, so Wednesday 2 September was followed by Thursday 14 September. Russia switched only in 1918, which is why its “October Revolution” happened in November by the modern calendar. Many Orthodox churches still use the old calendar for religious dates, which is why some celebrate Christmas on 7 January.

The riots that probably never happened

A popular story says angry crowds in 1752 marched under the slogan “Give us our eleven days!” Historians have found no reliable evidence of such riots. The slogan is best known from a satirical painting by William Hogarth, made a few years later to mock an election campaign.

What it still affects

Look at the UK tax year. It used to start on 25 March, an old New Year’s Day. To avoid losing eleven days of tax, the end of the tax year moved to 5 April. When 1800 skipped its leap day, the start shifted once more, to 6 April, where it remains today.

Sources & further reading

  1. 01Gregory XIII (1582). Inter gravissimas (papal bull reforming the calendar).
  2. 02Calendar (New Style) Act 1750, Parliament of Great Britain.
  3. 03Richards, E. G. (1998). Mapping Time: The Calendar and its History. Oxford University Press.
  4. 04Poole, R. (1995). 'Give us our eleven days!': Calendar reform in eighteenth-century England. Past & Present.
End of entry · 450 words