Informal in nature, relaxacons are generally small, ranging from 50 to 200 attendees. They are often invitation or word-of-mouth only. Frequently, they are a chance for the organizers of a large convention to get together with the other organizers and relax once the convention is over, and are held a month or more after the regular convention. Some large conventions use relaxacons as a reward for volunteers, and as a way of using up leftover supplies (much the way a dead dog party does). This can be seen as contributing to SMOF-ish behaviours, which some members of fandom can find off-putting.
Common fannish lore is that Midwestcon (started in 1950) was the first convention to label itself a relaxacon. However, the organizers first applied the term to the convention in 1966, according to Jack Chalker, and the term had been used in the Magazine of Fantasy and Science Fiction in 1957.