My first thought would be a doctor's appointment too. It would be either something that you're getting anesthesia for, though even still, you can usually have water up to 2-3 hours beforehand. Or it could be something like a blood glucose test (oddly specific example, I know lol) where they don't want you to eat or drink anything beforehand. Though I feel that you'd remember if you had either of those types of appointments scheduled...
Other than that, I have no clue! Would you have set a random water fasting time for a personal health reason or goal? Instead of a fast from food, a fast from water?