The day that Columbus set foot in America.
If you think he didn't, you're a moron. Yes. he wasn't the first European to set foot in America, that was Leif Erikson. However, Leif didn't make
monumental colonies spreading across the continent. This makes Columbus
the third person not from America to step there -- first was Leif Erikson, then Mansa Musa's predecessor, then Columbus.
And no, this day doesn't celebrate the deaths of
native Americans...
History fanatic, learn history.