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.