A Historical Lesson
...Whether an or a is to be used, depends on the sound with which the following word commences....
An is used before words commencing with a and i, and most words commencing with e, o, u, and h not sounded. A is used before all other words.
A must be used when the following word commences with the sound of u in unit; as, a unit, a ewe, a eulogy, a humour.
A must be used before one; as, a one-horse wagon.
A must be used before words commencing with h sounded; as, a hat, a hen.
That would also include a historic ocassion and a hysterical person.
The confusion stems from usage seen in the King James Bible, when all words having a French origin that began with h were pronounced with the h silent, such as "an hotel." This is no longer correct, even in England. So drop the n before historical.
(Source: Harper Dictionary of Contemporary Usage, 2nd ed. New York: Harper & Row, 1985: p. 1.)
(Illustrated Lessons in Our Language by G.P Quackenbos, is Available on CD at Lady's Maid Books.)