Many schools have payment plans created with certain payment methods (ex. auto-pay only) and specific due dates in mind.  However, the system doesn't stop a Smart Tuition user (rep. at the school, Smart Tuition rep., Parent Help Center, etc.) from choosing a date or method that should be "turned off."  If the setting is chosen to have a specific due date unavailable, such as the 25th for all payment plans as of the end of August, then that date shouldn't be available in the drop-down.