No sooner had I posted Historic vs. Historical: What's the difference? than a comment came in from lily_lemony:
Shouldn't it be 'The election of the first African American to President of the United States was AN historic event'?
This is one of those 'ear things.' In the US, we generally pronounce the 'h' in history, historic, and historical so we would say a historic. But in some dialects in the US and other countries, the 'h' in historic is not pronounced. In that case, it sounds better to say an historic.
Both the The Chicago Manual of Style and AP Stylebook prefer a historic. But I've seen it written as an historic in the Washington Post and various other publications. Do whatever feels more natural to you.
- Potato, potahto. A historic or an historic. Either way is fine.