Yes, this is a real holiday, passed by the U.S. Congress in 1988. Why May 25? Because it's the birthday of Bill "Bojangles" Robinson:
Bill Robinson was the most famous of all African American tap dancers in the twentieth century, and tap dancers around the world honor him and that art form every year.
Even though it's only celebrated for one day, there are performances and classes happening all month long throughout the country. (I was in a National Tap Day Show at the Old Town School of Folk Music this past Saturday.) Check out the International Tap Association website for the event nearest you.
And now for your viewing pleasure, a few great moments in tap history: