I absolutely and thoroughly hate it when people say the phrase "an historic". And I hate it for two reasons. One, it's wrong. And two, people only ever say it because they think it is proper English, which makes them extra wrong, and more stupid than the people they think they are correcting.
Now, let's start from the beginning. When do you use the article "a" and when do you use "an". If you're in elementary school, you were probably taught that "a" goes in front of words that start with consonants, and "an" goes in front of words that start with vowels. Now, this is almost true, but not entirely. However, I want to take a moment to point out the glaring problem that "H" is a consonant, which already makes the phrase "an historic" wrong at the elementary level.
The correct answer to "a" vs. "an" is that it doesn't matter what letter a word starts with: it matters what sound a word starts with. Most words that start with vowels have vowel sounds. "Apple" starts with "aaaa", which is why we say "an apple". And most words that start with consonants have consonant sounds, which is why we say "a flower".
Consider the letter F for a moment. The letter F is a consonant, but when you pronounce the letter, it starts with a vowel sound. That's why we would say a sentence like "I have an F." It's because, vocally, the letter is pronounced like 'eff'.
So to return to the original phrase, why do people say "an historic"? The reason is that the phrase originated in Britain. And, in Britain, the letter H is not pronounced when it begins a word. In fact, that is the reason that "herb" is pronounced like 'urb', and is why we say the phrase "an herb" (because the word starts with a vowel sound). However, most people do not speak with such accents when saying "historic". And if the word starts with a consonant sound, the proper article to use is "a".
This might be one of those minor grammar rules out there, but I felt the need to bring it up because it is one of those obscure artifacts that keeps rearing its head just enough to be worth explaining. It is also valuable because it can really open your eyes to think of words not in terms of the letter we use to spell them, but the kinds of sounds we use. And I think it is really trippy to have consonants that begin with vowel sounds when pronouncing its name, but that they don't have that vowel sound if the letter is part of a word.