HISTORIC: “famous or important in history,” as in a historic occasion
HISTORICAL: “concerning history or past events,” as in historical evidence
Thus, a “historic” event is one that was very important, whereas a “historical” event is something that happened in the past.
Buildings, villages, districts, and landmarks deemed important are often described as “historic” because they are significant to the past (in addition to being of or related to history.)
Societies dedicated to recognizing and preserving these things are called” historical” societies because they are concerned with documenting past history but not momentous in themselves.
The words were originally synonyms—with historic developing second as a shortened historical—but they began to diverge in meaning around the 18th century, and the difference has solidified over time. They are still occasionally mixed up, but the differentiation is now so well-established that using one in place of the other is likely to strike many English speakers as wrong.