Formally, the word historic begins with a consonant sound and so the form a historic is preferred in formal writing. However, many people prefer the form an historic in informal writing and speech for personal reasons.
This makes sense! It’s like lede vs lead. It’s really lede, but common usage has made lead acceptable. At least this is a better explanation and intuition than the poster below who simply asserts one thing based on the first letter without beginning to understand any context.