If it was a restaurant that you had to leave a CC for. Then 24 hours. With no charge. Otherwise, right up to the ADR. No penalty. But to be nice so someone can get a ressie. As soon as you know.
For most locations with a 24-hour cancellation policy, you actually have until 10 pm ET the night before (this is when the phone lines close). So if the reservation is for Tuesday (any time), you must cancel by 10 pm on Monday.
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