quoted from Lynch's guide to grammar and style, my emphasis on the important bit:
'One tricky case comes up from time to time: is it "a historic occasion" or "an historic occasion"? Some speakers favor the latter — more British than American speakers, but you'll find them in both places — using an on longish words (three or more syllables) beginning with h, where the first syllable isn't accented. They'd say, for instance, "a hístory textbook" (accent on the first syllable) but "an históric event." (Likewise "a hábit" but "an habítual offender," "a hýpothetical question" but "an hypóthesis.") Still, most guides prefer a before any h that's sounded: "a historic occasion," "a hysterical joke," "a habitual offender" — but "an honor" and "an hour" because those h's aren't sounded. [Entry revised 21 April 2006; revised again 10 December 2006.]'
Whatever sounds natural usually is correct.