April 21

Do you believe myths? Well, then, you're gonna love this one.
Rome was founded on this day by Romulus and Remus.
They were sons of the god Mars and a vestal virgin.
They had been set adrift in a little boat which shipwrecked (or thrown into the Tiber by their uncle), so they were raised by an enterprising lady wolf. Later they were introduced to humanity by a royal herdsman, who adopted them. All the stories seem to agree that Rome was founded on an April 21. After the twins founded Rome, they had a spat of some kind and Remus was killed.
I can't verify any of this, since the dates suggested were all Before the Christian Era.
They didn't keep as good records then, and it sounds like the records got a little muddled.
Anyway, Happy Birthday, Rome!