It's a(an) Honor to Witness a(an) Historical Event
Most of the time, it's clear which article to use, a or an: a before consonants, an before vowels. However, over the years, I've edited a number of manuscripts by well-educated authors who have slipped up when confronted by an H or an acronym. Here's what several reliable resources say:
Use the article a before consonant sounds: a historic event, a one-year term (sounds as if it begins with a w), a united stand (sounds like you). Use the article an before vowel sounds: an energy crisis, an honorable man (the h is silent), an NBA record (sounds like it begins with e), an 1890s celebration. (quoted from The Associated Press Stylebook; 2000)
Such forms as "an historical study" or "an union" are not idiomatic in American English. Before a pronounced h, long u (or eu), and such a word as one, the indefinite article should be a: a hotel, a historical study, a union, such a one; but an honor, an heir. (quoted from The Chicago Manual of Style, 13th ed. Chicago: The University of Chicago Press; 1982: p. 168)
The article a is used before aspirate h (eg, a historic occasion) and nonvocalic y (eg, a ubiquitous organism). Abbreviations and acronyms are preceded by a or an according to the sound following (eg, a UN resolution, an HMO plan). (quoted from American Medical Association Manual of Style: A Guide for Authors and Editors, 9th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 1998)Even an MBA graduate or a history professor can get a headache and need to visit an MD when tangling with a and an!