Consider the sound (not the spelling) of the following word.
Use "a" before all consonant sounds, including sounded "h", long "u" and "O" with the sound of "w" (as in one).; for example, a day, a week, a home, a hotel, a house, a unit, a union, a uniform, a one-week seminar, a CPA, a 60-day note.
Use "an" before all vowel sounds except long "U" and before words beginning with silent "H"; for example, an army, an evening, an invoice, an outlet, an umbrella, an heir, an hour, an honor, an "R" (pronounced "ar), an f.o.b. order (pronounced "ef of bee"), an 8-hour day.
Note: In speech, both a historic occasion and an historic occasion are correct, depending on whether the "h" is sounded or left silent. In writing, a historic occasion is the form more commonly used.
From The Gregg Reference Manual, Fifth Edition, Sabin.
