Crayfish are nocturnal. As in, they're more active at night while searching for food. So, obviously, they'd be most likely to bite your bait at night-time, given you're using a trap. That being said, crayfish often search for food during daytime hours too. Especially if they're very hungry. So, if you don't get good results from nighttime hours, don't hesitate to give the other time period a shot.