If a stranger approaches you with a raised arm on Thursday, don't be alarmed – it's just National High Five Day, an unofficial holiday during which participants raise money for charity by engaging in an all-day high-five-a-thon. The holiday began on the campus of the University of Virginia in 2002, when a group of students set up a stand giving away free high-fives in the middle of a quadrangle. "We discovered two things," Greg Harrell-Edge, executive director of the National High Five Project and one of the original UVA group, told PEOPLE. "One, it's real fun ...