You've come to this page because you've asked a question such as:
When is George Washington's birthday?
This is the Frequently Given Answer to such questions.
George Washington's birthday is (currently) on the 24th of February.
This will come as a surprise to most people. Many people think that it's the 11th of February, or the 22nd of February, or the third Monday in February. All are wrong.
Let's deal with the latter first. The third Monday in February is a United State national holiday, called "Washington's Birthday", as defined by the Uniform Monday Holiday Act 1968. It's not the actual birthday itself. It's when the federal government has a holiday in celebration of the birthday. (Anyone with any experience of birthdays will know that they don't fall on the same day of the week year after year.)
The other two are more related to the actual birthday itself. The 11th of February was the actual day of the year that Washington was born. But this was according to the Julian calendar, in force in the (then) British colonies at the time. According to the Gregorian calendar of the time, as in force elsewhere, the day was 1732-02-22.
There was a lot of confusion in the early years of celebrating Washington's birthday publicly, not least because Washington himself (at least for his 56th birthday) still used the 11th of February. But the switchover to the 22nd of February was largely accepted within a short time. The Society of St Tammany in New York City celebrated it on 1790-02-22, for example. By Washington's 60th birthday, even he himself was using the 22nd.
This is all fine and dandy. But the problem is that the changeover from Julian to Gregorian under British law, the law that applied to Washington at least from 1752 to 1776, doesn't actually keep birthdays static in the Gregorian calendar, for a whole class of people of which George Washington is a member. The Calendar Act of 1751 makes explicit provision for people's birthdays, specifically in respect of determing their age for legal purposes. It states that people born before the changeover should turn 21 on the same natural day as if the calendar hadn't changed, and have their ages determined accordingly for any other purposes. (This provision remained on the statute books for just under 200 years, being only finally consigned to legal history in 1948 by the Statute Law Revision Act of that year — an Act that cut much out of the statute books, affecting 750 Acts including parts of the 1558 Act Of Supremacy and parts of Magna Carta.)
So according to U.K. law, George Washington turned 21 on 1753-02-22. But he is required, as are everyone (including Sir Isaac Newton), to consider years to be longer then they really physically are when measuring their lifespans. The Act requires that everyone born before the changeover date continue to determine their ages using the year lengths of the Julian calendar.
(It's ironic that Washington himself railed against the switchover for supposedly shortening his life. This is widely considered to be a popular misunderstanding of the Act by people at the time. But for Washington and his contemporaries, it in fact really did shorten lifespans, albeit not in the way that Washington and others thought it did. Years went by more slowly for him and his generation, thanks to the Act, than they did for anyone born later. If he'd been born later, the same lifespan in days could have been measured as a larger number of Gregorian calendar, shorter, years.)
So this means that as the Julian and Gregorian calendar get further out of step, as they do three out of every four centuries, the birthdays of all such people gradually advance down the Gregorian calendar. In the 18th century, February the 11th Julian was February the 22nd Gregorian. But in the 19th century it was February the 23rd Gregorian, thanks to a lack of a Gregorian calendar leap day in 1800, and in the 20th century it became February the 24th Gregorian, thanks to a lack of a Gregorian calendar leap day in 1900.
Thanks to the Statute Law Revision Act 1948, this provision of the Calendar Act is now repealed. (Even had it not been, February the 11th Julian is still February the 24th Gregorian in the 21st century. The two calendars didn't slip in 2000, because the Gregorian calendar made that a leap year, like the Julian. The next slip is in the 22nd century, where February the 11th Julian will progress to February the 25th Gregorian.) But this only serves to make the situation even messier:
The Statute Law Revision Act 1948 is an Act of the United Kingdom Parliament. So it's debatable whether it repeals the effects of the Calendar Act in the United States. This is a complex issue in public international law.
If it doesn't repeal the Calendar Act, then George Washington's birthday is still subject to the unamended Calendar Act, and will continue to progress one day three out of every four centuries.
The Statute Law Revision Act 1948 just cleaned lots of wording off the statute books, to make them shorter. It didn't introduce new provisions, merely deleted lots of text from the statute books. As was explained in the House of Lords at the time, it was not the intent of Parliament to enact retrospective changes with the Act. (One of the controversies surrounding the Act in the popular press was the supposed effect that it would have with the repeal of the Act of Attainder of James, Duke of Monmouth, for High Treason.) The Lord Chancellor explained in the Lords that it was not the intent of the Act "to introduce substantial or serious modifications in existing law".
Given that, the 1948 Act cannot be considered to act retrospectively, so the effects of the law, as it applied to George Washington's birthday until the 20th century, stand unchanged. Nothing futher changes after 1948, and the rules for birthdays presumably revert to the usual once per Gregorian calendar year for George Washington and his contemporaries. But the changes up until 1948 stand. So George Washington's birthday continues to fall on the same Gregorian day as it did in 1948, which was February the 24th.
It seems that the Lord Chancellor in 1948 forgot, in amongst all of the hullaballoo over Magna Carta and some treasonous duke, that there are still a scant few people born before 1753 whose birthdays are still celebrated.