I’ve always thought that “a historic” sounds correct but never used it, having only seen “an historic” in publications.
I’m very conscious of exactly what I’m writing because I have that internal conflict. It will haunt me through life, I fear.
If one pronounces the H, then it should be “a historic.” If one pronounces the word with a silent H, then it should be “an historic.”
Personally, I say “an historic occasion,” but, then again, I’m quite the snob. : )
BTW, in the U.S. Constitution, the Framers wrote “an uniform rule of naturalization.” Apparently, they pronounced it “OOH-neh-form” back then.