It depends on the date of the party. Some days have ticket availability on the day of the party. I'm not exactly sure what would make one day more favorable than another (like at Halloween when the Friday shows tend to go fast), but I've always had the option to buy tickets the day of the party the days I've been there in early December, so I wouldn't worry about rushing it too much...my advice would be to plan for a party early in your trip, and if it sells out before you can buy tickets, grab tickets for a later day in your trip.