I dont like alot of workouts that are designed to be done on specific days for this reason. If this happens often it will hamper your progress, I think ideally the program should be designed to avoid this type of problem. That is why I do a program with day 1 day2 etc, not specific days of the week. And remember, your nervous system could still be shot, I wouldnt lift light just to get a workout in, because then you are going away from what you would have done. But, as far as soreness goes, I think the general rule is, as long as your not so sore that it affects your performance on that day then you are ok... I have heard this before, dont know if it holds any truth..