Directly from the Disney World Dining FAQ page:
According to the credit card guarantee policy, I must cancel my reservation at least one day in advance. What does this mean?
At restaurants with the credit card guarantee policy, all cancellations should be made at least 1 day in advance of your reservation. For example: if your reservation is on Thursday, you must cancel your reservation no later than Wednesday end of day. Cancellation fees will be assessed for cancellations made on the day of your reservation.