If you graduated from college, your graduation is now historical. If you never expected to graduate because you spent your wayward youth wearing lampshades over your head at parties, perhaps that graduation would count as historic for you alone. These words are not used interchangeably.
- The abolition of slavery was a historical event.
- His participation in the Boy Scouts was historic.
- The formation of the European Union was historical.
Historic is used specifically for events that have been important. The San Francisco earthquake of 1906 was historic; the stock market crash of 1929 was also historic. Anything historical has to do with some particular time in the past. That pile of dusty papers in one’s study from 1998 are anything but historic; they are merely historical. No matter how momentous and surprising your high school graduation may have been, it doesn’t count as historic.
You may hear people use the article an in front of historic (or historical). This is a result of British English speakers’ tendency to drop the initial h in words like hour and honor. In American English, you can use a before historic / historical event, because we pronounce the initial h.