It is no longer accepting "an." That is wrong and ridiculous. "A" and "an" are both correct, even if "a" is preferred. http://www.oxforddictionaries.com/words/a-historic-event-or-an-historic-event. I'm reporting this.
It depends on whether or not you pronounce the 'h', not whether or not it is there. Most commonly, you would pronounce the 'h' in 'historical' (think of the word 'history'), so it would use the 'a'. The same is true for words like 'user' that have a clear 'j' sound in the beginning, warranting an 'a'. But you would use "AN unusual thing" because there 'u' is pronounced as a pure vowel. Hope this helps.
I think they may actually be right. I was always told that you use 'an' before a silent h. Words like hour or honor. Words like history, or historical, would use the article 'a'
edit to add- I just looked it up out of curiosity and it looks like both ways are acceptable, but it comes down to the way you say a word