Discussion Forum—A Way with Words, a fun radio show and podcast about language

I've noticed over the past few months, with the Obama Presidency and all that's going on politically that several people/news commentators have called it a "historical occasion" or a "historical day". This seems really odd to me. I always thought you would use the terms "historic occasion" or "historic day". Could you please explain the difference between historic and historical and when you should use each.