They also made it a bit clearer on their About/FAQ page - not sure why these bullet points weren’t included on the actual ticket page though - would have made it easier for people to find if it was all in one place:
Why do I need to select a start date for my Fear Pass?
Due to high demand and in an effort to help preserve the event’s unique guest experience, Fear Pass purchasers will be prompted to select a start date for their pass. We strongly encourage guests to purchase their Fear Passes early as nights are expected to sell out.
Why are some dates that are supposed to be a part of my Fear Pass entitlement not available as a start date?
Due to the event’s popularity and high demand, Halloween Horror Nights event nights are expected to sell out. If one of the applicable dates for the Fear Pass selected is not available as a start date, that means the event is sold out for that evening.