"Historic is a word which implies judgment, since by definition it describes something significant. But . . . historical is an essentially neutral term, describing anything which occurred in the (distant) past." (Philip Gooden, Who's Whose: A No-Nonsense Guide to Easily Confused Words, Walker & Company, 2004)"
If something is "historic" it is of importance in history.
If it is "historical" it is simply something that happened in the past.
The two can have completely different implications -- if something is historical it is in the past -- gone, and of no further significance. But an historic event is still of great importance or significance.
The OED gives this as the current sense of historic:
"esp. Forming an important part or item of history; noted or celebrated in history; having an interest or importance due to connexion with historical events. (The prevailing current sense.)"