When checking online for a Fantasmic DP, you need to type out the date like this 09/01/2011. Double check the date before you confirm and give your CC information.
If you select a grey date from the calendar, click on the change search options button or type in a date that Fantasmic isn't showing it will revert the date to tomorrow's date.
It's possible that there isn't a Fantasmic DP available for the date you want as there are few available and they have been available for quite a long time now.