As noted, possibilities include special events or walking a ressie. There is also just usual randomness -- what may be happening, particularly if this is for a 7-month out reservation, is that the full middle-of-the-week day is just the first one to go full while the other days around it are also very close to being full. That one day just has more check-ins than check-outs, while the nearby days still have more check-outs than check-ins leaving them still shown as open.