Using “a” or “an” before a word starting with “h” usually depends on whether the “h” is sounded or unsounded. We say “a house” but “an hour”. Usage has differed over time and differs between countries and also dialects.
Before the twentieth century, the letter “h” wasn’t usually sounded in English. Thus any word starting with “h” took “an” as the indefinite article rather than “a”. This was at least partly due to the influence of French on the English language, which was very strong at certain times in history. It should be noted that “h” is still always unsounded in French.
When the British sounded their “h’s” more and more, words starting with “h” increasingly took the indefinite article, “a”, in line with the rule of using “a” when the next word starts with a consonant sound rather from a vowel sound.
There is, however, a group of words of three or more syllables with the stress on the second syllable, such as historic, historical, hypothesis, hysterical, habitual, harmonica and hereditary, where people tended to still use “an” rather than “a”. The “h” is less well sounded in these words compared with certain other words starting with “h” where the stress is on the first syllable, such as history, histogram, hypothetical, holiday and hemorrhoid, or on the only syllable such as hand, host and hymn. Thus “an historic” is still often used.
The word “haphazard” is an interesting exception. Here, the stress is on the second syllable but we hardly ever see, for example, “an haphazard event” even though it fits into the same category of words as “historic” etc above. This is perhaps because the first syllable of “haphazard” is actually quite strong even though the stress is on the second syllable.
The stress on the second syllable of words like “historic” meant these words have been much slower to move away from the use of “an” to “a” after the letter “h” generally became sounded in the English language. The move has been slower in Britain than in the US where “an historic” is less common. Canada and Australia have been somewhere in the middle. Reasons for the slower change in Britain is perhaps due to the tendency to still drop “h’s” in many areas.
Also, British dialects tend to use a softer and quicker pronunciation of the first and third syllables of words like “historic” compared with American dialects. In other words, the “h” sound in “historic” in American English is stronger than in British English and is thus more likely to have “a” preceding it than “an”.
As the letter “h” is now sounded in English, the trend is away from “an historic” to “a historic”. Some sources such as the American Heritage Dictionary say “an historic” is outdated. What is acceptable is often determined by general usage. In 2008, a Google search found 5.43 million web pages using “a historic” (68%) and only 2.58 million using “an historic” (32%). By 2010, the proportion of pages using “a historic” had risen to 71%. By 2013, it was 75%.
All in all, it appears that “a historic” will win the day, although “an historic” will still no doubt be used where the “h” sound is weak. This could especially be the case in certain dialects of spoken English – “an ’istoric” can be easier to say than “a historic”, especially if talking quickly. Otherwise, “a historic” seems to be becoming the norm.