As far as I know, all rabbits (domesticated or wild) are most active at both dawn and dusk - they are crepuscular. The reason for this behavior is supposedly to avoid predators. So either head out at dawn or in the evening, for best chances of spotting them.
Additionally (according to Wikpedia), cottontails dislike windy days since it has an impact on their hearing and prefer to stay at home because of that. So it would be a bad idea to go rabbit spotting when it's windy.