I hate it when I hear things like “an historical moment” said in an American accent. I hate it more when people actually cite those instances as an exception to the “a-an” rule. The reason why you see “an” in front of words starting with “h” is that in British English that “h” is silent. That’s it. If you’re reading something out loud, you can say “an historical” or “an happy” or whatever. I guess. Otherwise, stop it! (And to be honest, I try to read those things with a British accent. “an ‘istorical”. “an ‘appy’'.)
There. I feel better now.