It depends on whether you had to give a credit card number or not when you made the reservation.
If there was no credit card required you can cancel at anytime. As a courtesy it is nice to cancel as soon as you know you won't be needing the ADR.
If you had to give a credit card number as a guarantee or it was a prepaid meal (CRT or dinner shows) then you must cancel 2 days before your reservation or you will be charged a no-show fee of $10 or $20 per person depending on the restaurant or you will forfeit the entire pre-payment at CRT or the dinner shows.