Have you ever Googled George Washington? You may have noticed that he often has two birthdays listed. Believe it or not, both dates are actually correct. Here's why.
George Washington's official birthday is listed on both February 11, 1732, and February 22, 1731, and it can all be traced back to this exact date (October 4) in 1582. On this date, Pope Gregory the XIII introduced the Gregorian calendar, which you may notice is named after him and is the same calendar that we still follow today. You see, up until 1582 almost the entire world was following the Julian calendar, implemented by Julius Caesar back in 46 B.C. The Julian calendar was almost exactly the same as our modern calendar, except for the fact that it added 3 too many leap days every 400 years. So, by the time 1600 years passed, over ten extra days had been added to the calendar, which made it unsynchronized. Catholics were particularly aware of this, because Easter no longer fell at the right time each year (historically, Easter is supposed to match up with the Vernal equinox, but the Vernal equinox didn't happen on the right date, which is how they knew things were out of whack). To resynchronize the calendars, they needed to delete the extra days that had been added to the Julian calendar... and they needed to drop 3 leap days every 400 years.
So they did just that. The Pope actually struck 10 days from the calendar and October 4, 1582, was immediately followed by October 15, 1582. To account for the leap days, they created our current system. Every four years is a leap year, however, a new rule was implemented stating that years divisible by 100 would be leap years only if they were divisible by 400 as well. (ie. 2000 was a leap year, but 1900, 1800, and 1700 were not).
So where does George Washington come in? Hang on to your horse, because things are about to get a little more confusing...
By 1582, Britain and her colonies were no longer Catholic. They had shifted to Protestantism and had no desire to listen to what the Pope and Catholicism had to say about changing the calendar. They continued to use the Julian calendar. To make matters worse, calendars across Europe had gotten mixed up during the Middle Ages and the result was that not every country recognized January 1st as the start of the year. Britain was one of these countries. Up until 1752, Britain recognized March 25th as the first day of the year. This became a complete catastrophe when it came to setting dates and records for trade and recording history. When looking at old documents, you may see years written like this: "1731/2," accounting for the difference in calendars.
In 1751, Britain decided it was time to join the majority of countries in the world and recognize January 1st as the first day of the year and also recognize the Gregorian calendar as the official civil calendar. First, the British government ordered that all citizens of the crown recognize that 1751 would end on December 31st, thus making January 1st the first day of 1752. Also, like the Catholics did in 1582, it then deleted 10 days from the calendar. Sept 3-13th of 1752 were completely struck from the record.
So, when George Washington was born, the date at that time was officially February 11, 1731, according to the Julian calendar. At the time of his birth, Britain still started the new year on March 25 and had yet to delete the 10 extra days from the calendar. However, most historians today now record time retroactively according to the Gregorian calendar, so this is why you will also see George Washington's birthday listed as February 22, 1732. This date is adjusted to recognize the adoption of January 1st as the first day of the year and the adoption of the Gregorian calendar.
Fun fact: Greece didn't adopt the Gregorian calendar until 1923.
Even more fun fact: Saudi Arabia just adopted the Gregorian calendar two days ago. Many experts believe that they did this because the Gregorian calendar will require them to delete the extra 11 days that have been added on to the Julian calendar. They are currently hurting for money and by deleting these 11 days, it means they won't have to pay their government workers as much this year.