Usage Note: Historic and historical have different usages, though their senses overlap. Historic refers to what is important in history: the historic first voyage to the moon. It is also used of what is famous or interesting because of its association with persons or events in history: a historic house. Historical refers to whatever existed in the past, whether regarded as important or not: a minor historical character. Historical also refers to anything concerned with history or the study of the past: a historical novel; historical discoveries. While these distinctions are useful, these words are often used interchangeably, as in historic times or historical times.
I was curious about this because a lot of folks are referring to the "historical DNC convention." My understanding has always been that "historical" referred to things that happened in the past, and "historic" was the sense that something was a significant event in history. Apparently, dictionary.com, quoted above, says that the two words can be used interchangably. Merriam-Webster seems to agree.