Ghoti.

Can someone explain why it's often written as "an historic event" instead of "a historic event"?

[edit] Discrimination between a and an
The form "an" is always prescribed before words beginning with a silent h, such as "honorable", "heir", "hour", and, in American English, "herb". Some British dialects (for example, Cockney) silence all initial h's (h-dropping) and so employ "an" all the time: e.g., "an 'elmet". Many British usage books, therefore, discount a usage which some Americans (amongst others) employ as being a derivative of the Cockney.[citation needed] The reason is that the indefinite article a is pronounced either of two ways: as a schwa or as the letter itself is pronounced, "long a" (actually a diphthong, /eɪ/). Some words beginning with the letter h have the primary stress on the second or later syllable. Pronouncing a as a schwa can diminish the sound of the schwa and melt into the vowel. Pronouncing it as a "long a" does not do this, but as the pronunciation cannot be prescribed, the word is spelled the same for either. Hence an may be seen in such phrases as "an historic", "an heroic", and "an hôtel of excellence" was the by-line in an advertisement in a New York City newspaper[citation needed].

Merriam-Webster's Dictionary of English Usage is more descriptive than prescriptive, but it advises, "You choose the article that suits your own pronunciation." Theodore Bernstein gives the straight vowel-sound vs. consonant-sound explanation but allows that one should indeed say "an hotel" if they think hotel is pronounced otel. [1]

Such was also the case for some other words which take the place of the article. "My" and "thy" became "mine" and "thine", as in "mine uncle". This usage is now obsolete.

The appearance of an or a in front of words beginning with h is not limited to stress. Sometimes there are historical roots as well. Words that may have had a route into English via French (where all "h"s are unpronounced) may have an to avoid an unusual pronunciation. Words that derived from German however would use a as the h's would be pronounced. There is even some suggestion that fashion may have had some influence. When England was ruled by a French aristocracy, the tradition may have been to exclusively use an, while when Britain was governed by a German-based monarchy the tide may have changed to a.

Further, some words starting with "u" (like "ukulele" or "user") have a preceding "a" because they are pronounced as if beginning with an initial "y" consonant.

I may be wrong, but I believe it stems from English dialects dropping the "h" while speaking, and their emphasis is on the second syllable instead of the first like North American dialects.

I use 'an' with historic - in English it's perfectly acceptable. Doesn't help that I pronounce "historic" as "istoric" though

What REALLY p****s me off is when people say that a person was "hung".

It's HANGED Kakdammit! HANGED! Meat is hung, people are hanged.

and thats why its called a hanging... not a hunging...

I use 'an' with historic - in English it's perfectly acceptable. Doesn't help that I pronounce "historic" as "istoric" though

What REALLY p****s me off is when people say that a person was "hung".

It's HANGED Kakdammit! HANGED! Meat is hung, people are hanged.

I am hung.