Why is Friday 13 unluky?

It's somthing to do with the knights of the templar ( I think), They were all assassinated on the same day (Friday the 13th) and at the same time, quite a feat considering the times! If you watch the Da Vinci code DVD it explains about it on that!! I hope this is helpful or at least a little intersting!! I'd google to get more info tho!! xx