Well I voted never, and then saw the next option. Obviously if it's the first word of a sentence it should be capitalized (any word should be, I can't think of any exclusions to that rule except perhaps a persnickety business which insists on going by a lowercase spelling - even then, it wouldn't be technically correct). Fall, as a season, is not a proper noun.
You would capitalize it as part of a proper noun - name of something - OR if it were used in a headline or title, you'd follow the headline and title capitalization rules.
We don't capitalize winter, spring, and summer, so why capitalize fall? Now if it's being used properly as in it's part of a name of an event, then it should follow proper noun rules, and of course when its at the start of a sentence.