We go to WDW at least twice a year. I have often cancelled very late in the evening, but before midnight, and never been charged a fee. If you find that, on the day of your ADR, you need to cancel because of illness or some other unfortunate event, it is possible to do without being charged. On the very few occasions when I needed to cancel an ADR on the same day, I have gone into the ADR that I need to cancel and modified it to a date in the future, and then cancelled the future date. I know it is often suggested that you go to the restaurant and explain why you can't make it and hope for dispensation, but that can be hard to do if you are at the resort with a sick family member.