Historically:
* "H" was often silent: In the past, the "h" in many words, including "historic", was often silent. This is why we still say "honor" and "hour" with a silent "h". Therefore, "an historic event" would have sounded perfectly natural.
Modern Usage:
* "H" is pronounced: As pronunciation evolved, the "h" in "historic" became pronounced. This shift led to confusion about whether to use "a" or "an".
* Grammar Rules: The traditional rule is to use "an" before words starting with vowel sounds and "a" before words starting with consonant sounds. However, "historic" has a consonant sound, making "a historic event" grammatically correct.
Contemporary Debate:
* Prescriptive vs. Descriptive Grammar: This situation highlights the difference between prescriptive grammar (rules set by authorities) and descriptive grammar (how language is actually used).
* Evolving Usage: Some argue that "an historic" has become so common that it's now acceptable, even if it doesn't strictly follow traditional rules. Others hold to the traditional rules, emphasizing the consonant sound of "h".
In Summary:
There's no single "historic" moment when "historic" started being treated as if it began with a vowel. It's an ongoing evolution, with some embracing the newer usage and others adhering to traditional rules. The choice ultimately depends on personal preference and the context of the writing or speech.