I think they cancelled Lincoln’s Birthday (February 12) to make room for MLK Day. Then they changed Washington’s Birthday (February 22) to President’s Day so as to include Lincoln and the racist Lyndon Baines Johnson.
I’m glad I live in the free state of Alabama. We still have Robert E. Lee’s birthday as a state holiday.
That probably is when they consolidated them; when I was in elementary school we had the birthdays of Washington and Lincoln as separate days off (and MLK Day hadn’t happened yet).