A historical event or an historical event i go for the second.
It depends on (a) the dialect of English and (b) the age of the speaker. Here's a bit taken from the article a historical? an historical. The link is provided below.
Read more here: http://www.grammartips.homestead.com/historical.htmlan was the preferred usage before an unaccented syllable beginning with h. This is what the grammarian's grammarian, Henry Fowler, has to say on the subject:
. . . an was formerly used before an unaccented syllable beginning with
h and is still often seen and heard (an historian, an hotel, an hysterical scene, an hereditary title, an habitual offender). But now that the h in such words is pronounced, the distinction has become anomalous and will no doubt disappear in time. Meantime, speakers who like to say an should not try to have it both ways by aspirating the h. (1)**