I can't tell you how much that messes with me still every time I see initials for this place.
It's Martin Luther King, Jr. , the civil rights leader, Day here in the U.S.
His name is also all over street names and things, so the initials of MLK get used in the US a lot instead of saying "Martin Luther King Jr. Ave/Boulevard/Street/Etc." each time.
I'm so used to MLK meaning Martin Luther King Jr. that when I see it pop up in the context of a Disney messageboard I always feel a very large disconnect o_O
EDIT: But awesome for you lucky people that get the day off