Why Is Friday the 13th Considered Unlucky?

I've always wondered about this one.  I know it happens at least once a year but why the hell is it a bad/unlucky day?  Personally, I cannot recall ever having had bad luck on this date.  However, that's just me.  If you or someone you know has been unlucky on a previous Friday the 13th, or had a uniquely bad experience as a result of this spooky date, I'd love to hear about it in the comments.

That said, I hope your day is full of luck and good experiences!  Happy Friday the 13th, y'all!