I have been going to Disney and attending
MVMCP since the 1990s; also a former CM.....every party has not "historically" sold out, even when tickets were only $20.
Disney does not release data, however those who worked the parties can say with certainty that Fridays and Sundays had more people in the parks from 7 PM to 12 AM than a Tuesday, or even a Thursday.
Additionally, Disney has been known to raise the CAP on tickets sold when they sell out quickly.