A Google search will show more uses of a historic than an historic, but in speech, “an historic event” is the more idiomatic.
To repeat something I wrote in a comment,
“An historic” is idiomatic when the words are run together and the stress falls on the second syllable of historic. The use of “an” becomes self-conscious and unidiomatic when the speaker pauses after the “an” and then puts the stress on the first syllable of historic.
Some speakers tend to use historic and historical interchangeably, but a useful difference exists.
The word historic has the sense of uniqueness. An historic event is one that stands out as having had a significant, history-changing impact. The Battle of Waterloo was an historic event. It stopped Napoleon’s wars of conquest. The Seneca Falls Convention in 1848 was an historic event. It mobilized the women’s suffrage movement in the United States.
The word historical, on the other hand, may be applied to any event that occurred in the past. The Battle of Waterloo was both historical and historic.
On the other hand, if you wish to say that something is not historical, you can use either ahistoric. or ahistorical to mean the same thing: “not concerned with or related to history.”
For example, a political leader who repeats the mistakes of his predecessors may be said to have an ahistoric attitude towards governing. A novel based on an historical person or event may nevertheless be ahistorical if it interprets the character or event in a way to contradict the known facts.
Another, useful, if unlovely, “history” word is historicity. Ex. Some people deny the historicity of the Nazi Holocaust of the Jews and Gypsies.