How about when the first space shuttle was landing and nobody knew exactly where it was going to land. I told the office that I needed parts for my car but the flight was delayed due to the space shuttle landing.
Just a month ago I said I had an introductory camp for my upcoming study at the university, and I couldn't come that Saturday to work ... actually I could work after all, because I decided not to go to the introductory camp... but I was tired (already had had five days of introduction at the university) so I had a pretty good reason after all actually.
The most classic reason, ofcourse, is that you're sick and have some 24hour flu or something. Usually, I take work off when I'm sick and that's when I really am sick. If I need to take day off, I usually just take it off in advance.