It depends on the date.
From what I've gathered on this site and others, is that only the Halloween night ones sell out far in advance (the 31st). However, I believe the weekend night ones get harder to get as well as the time approaches.
Personally, I bought mine for the 20th already, but for the date I probably could have waited. I think, depending on the date of course, that you could wait until the Summer if need be, but maybe June/July instead of August just to be safe?