Both occur, but "a historic" is standard choice in modern English.

Since most speakers pronounce the h, it takes "a" not "an".

Contrast to "an hour", where the h is silent.

Both are being used and neither is incorrect. “An historic” used to be more prevalent in British English until around 1990, but “a historic” has become the standard choice now: https://books.google.com/ngrams/graph?content=a+historic%2Ca...

It depends on if you're speaking British or American English.

The British pronounce it as "istory," so they use "an" before.

An would always be wrong here, in the U.S. at least.

Depends on whether or not you pronounce the H.