From what I can find out on the net, either is correct. "An historic event' has always been my choice because it sounds better to my ear and looks better on paper. To my ear, the "H" in Historic seems to be a vowel sound so I use "An." Following that logic I'd use An with "An hysterical event" but it doesn't sound right. Maybe I need to repeat it over and over a few times. What am I missing here? I'm working on a tale based on "An historical event" right now and the usage of "A" or "An" popped up.