I would say that most post openings as they come up, some more timely than others. Sometimes the websites are more (or less) accurate than the postings on Search, etc. There are always some positions that appear at the fairs (and some that disappear), but most will be known before the fair.
Almost all are posted when the are known. Waiting to the fair does not help the school or the recruiter at all. New openings can occur as the recruiting season moves on if existing staff get jobs at other schools.