They can sell out when park levels are a 9/10 crowd
Spring break for some dates, major holidays (Christmas week to new year eve, etc) have sold out for a day ep
When the 3 hotels are sold out and the other two hotels also, it is possible the eps sell quickly
I have not seen both parks sell out for the same dates
One park usually still have some eps to sell
It’s difficult to predict
Ep prices increase based on the crowds in the park
Some will buy the ep and not really need them if crowds are manageable