When I was young, some states celebrated George Washington’s Birthday on February 22 and Lincoln’s Birthday on February 12. So what changed this? The Uniform Monday Holiday Act. This was enacted by Congress in 1971. It moved some holidays from their fixed dates to designated Mondays. Presidents’ Day is only known as that in some states. It is still considered George Washington’s Birthday. When the law was passed in 1968, it was designated that the Monday that falls between February 15-21, the 3rd Monday, would be George Washington’s Birthday. The reason for the change was to provide more 3-day weekends for federal employees.
Lincoln’s Birthday was never a federal holiday. It was a legal holiday in some states. So maybe instead of calling it Presidents’ Day, moving the apostrophe to make it President’s Day would be more accurate since on the federal level, it is still George Washington’s Birthday.