There is no August 21 event of significance. Most likely it is just a day when the number of check-in's outnumber enough the number of check-out's. You are less than five months out and should not assume that seeing all the days around Aug 21 still avaialble means there is plenty of availability for all those other days. What you could be seeing is simply the first full day among others that are also close to being full. Waitlists are not guaranteed to come through but getting one day is usually easier than getting more and if you are going to waitlist, you should likely reserve now, via two reservations later to be linked, all the other nights you want because those may also disappear soon.
