Dates and Days

Day of the week

What day of the week was any date

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Where and when the calendar changed, and the weekday on each side of the change
WhereLast Julian dateFirst Gregorian dateDates skippedWeekday ran
How this was counted
Weekday— Day of year— ISO week— Called there— From today—

The Julian calendar and the 1582, 1752 and 1918 switches are tested on every build, including 61,000 round-trip dates. How it is checked

Enter a date to get its weekday, and choose a place under Asking about to see what the date was called there at the time. When Europe changed calendars, dates were dropped from the calendar and the week ran on unbroken: Britain went to bed on Wednesday 2 September 1752 and woke on Thursday the 14th.

The eleven days that never existed

Adding a Julian leap day every fourth year, without exception, overshoots the solar year by about eleven minutes a year. Over sixteen centuries that came to ten days of drift against the seasons. In 1582 the new Gregorian calendar dropped those ten days and brought in the century rule that keeps the drift from coming back. Spain, Portugal, Italy and Poland changed that October: Thursday the 4th was followed by Friday the 15th.

Other countries took longer. Britain and its colonies waited until 1752, when the gap had grown to eleven days. Russia held out until 1918 and skipped thirteen. Greece was last, in 1923. So for any date between 1582 and 1923, what the date was called depends on the place, which is what the Asking about list above is for.

Old Style, New Style, and why Washington has two birthdays

England had a second quirk: until 1752 its year number changed on 25 March, where ours changes on 1 January. A date in January, February or most of March therefore carried what we would now call the previous year’s number. By the calendar in use where he lived, George Washington was born on 11 February 1731; by ours, on 22 February 1732. People at the time found this confusing too, which is where the “1731/32” style of writing such dates comes from.

Choose Britain and enter a date before the 1752 switch, and the page gives both labels, with the older year number where it applies. A date quoted without saying whether it is Old Style or New Style can be out by both eleven days and a year.

Why Sweden is not in the table

Sweden planned a gradual change, dropping the leap days from 1700 to 1740 one by one. After the first it forgot, and for twelve years its calendar matched no one else’s: a day ahead of the Julian world and ten behind the Gregorian one. In 1712 it added a 30 February to get back to Julian, and it switched properly in 1753. No pair of dates in the table could describe that, so Sweden is left out.

Questions

What day of the week was I born?
Enter your date of birth above and the Weekday readout gives the answer. The weekday cycle has never been interrupted anywhere, so there is no ambiguity and no convention to choose. Only the date itself can be ambiguous, and only for a date before 1923 in a country that had not yet adopted the Gregorian calendar.
Why did Britain skip eleven days in 1752?
Because the Julian rule of a leap year every fourth year, with no exceptions, is about eleven minutes a year too generous, and by 1752 that had added up to eleven days of drift against the seasons. The Gregorian reform of 1582 removed the accumulated error and added the century rule that prevents it recurring. Britain adopted it 170 years late, so Wednesday 2 September 1752 was followed by Thursday 14 September. The dates 3 to 13 September 1752 never existed in Britain or its colonies.
Did anyone lose a day of the week?
No. Days were dropped from the calendar, and the week carried on without a break. Every switch in the table runs from one weekday directly to the next with nothing missing. That is why the weekday answer above needs no region selected: it is the same everywhere, for every date, whichever calendar people were using at the time.
Why do some historical dates have two different years?
Because England began its civil year on 25 March until 1752, so dates in January, February and most of March carried the previous year's number. George Washington was born 11 February 1731 by the calendar where he lived and 22 February 1732 by ours: one day with two labels, eleven days and a year apart. The convention of writing such dates as 1731/32 exists because contemporaries found it confusing as well. Historians call the contemporary label Old Style and the modern one New Style.
Why is Sweden missing from the table?
Because no single pair of dates describes what Sweden did. It decided to convert gradually by omitting every leap day from 1700 to 1740, skipped one, then forgot, and spent twelve years on a calendar that agreed with nobody: one day ahead of the Julian world and ten days behind the Gregorian one. In 1712 it gave up and added a 30 February to return to the Julian calendar, then converted properly in 1753. Any single switch date for Sweden would be wrong.