A calendar that drifted
The calendar on your wall is not the one people used for most of history. For about sixteen centuries, much of Europe followed the Julian calendar, introduced by Julius Caesar in 45 BCE. It was a clever design, but slightly too long: its average year of 365.25 days is about 11 minutes longer than the actual solar year of roughly 365.2422 days. That sounds tiny, yet it adds up to about one day every 128 years.
Why it mattered
By the 16th century, the drift had reached about ten days. The spring equinox, which the church had fixed on 21 March in the 4th century, was by then falling around 11 March on the calendar. That threw off the calculation of Easter, which is tied to the equinox, and the seasons no longer matched the dates.
The 1582 reform
Pope Gregory XIII ordered a reform, designed by astronomers and mathematicians of the time. It did two things:
- It removed ten days to bring the calendar back in line with the seasons.
- It changed the leap year rule so that the drift would not return. Century years are leap years only if they are divisible by 400, which skips three leap days every 400 years and makes the average year 365.2425 days.
In the countries that adopted the reform first, including Italy, Spain and Portugal, Thursday 4 October 1582 was followed directly by Friday 15 October 1582. The days of the week continued without a break, so nobody lost a weekday, only ten dates.
Different countries switched at different times
Not everyone accepted the reform at once. Protestant and Orthodox countries were slow to adopt a papal decree, and the change happened country by country over centuries:
- Britain and its colonies switched in 1752. Wednesday 2 September 1752 was followed by Thursday 14 September 1752, which removed eleven days because the gap had grown by then. England also moved the start of the year from 25 March to 1 January in the same period.
- Russia adopted the Gregorian calendar in 1918, after the revolution.
- Other countries changed at various dates in between.
Old Style and New Style dates
The overlap means some historical events have two dates. George Washington, for example, was born on 11 February 1731 under the Julian calendar then used in the British colonies, which corresponds to 22 February 1732 in the Gregorian calendar we use today. Historians often write "Old Style" and "New Style" (or O.S. and N.S.) to avoid confusion.
What this means for date calculators
Calculating the weekday of a historical date depends on which calendar applies in that place at that time. Our day of the week calculator uses the Gregorian calendar throughout and only accepts dates from 1583, which avoids most of the confusion. For records from a place that switched later, such as Britain before 1752, the local weekday of an event may differ from what the calculator shows.
To learn how the Gregorian leap rule works in practice, see our guide to leap day birthdays.
This article is a general overview. For research that depends on exact historical dates, consult specialist sources.