If you have a choice, I suggest that you fish for catfish just before dark until about midnight. This seems to be the best time, especially for channel cats.
However, anytime you can get bait in the water you have a chance to catch catfish.
As far as time of year, spring seems to generate the best overall bite. Just before spawning they will come into shallow water to feed and will be most likely to hit your baits. May through August, depending on the weather, are good times.
It is easier to judge the spawn in impoundments because the water temperature is more constant. In a river the spawn can be very tricky to predict, especially if the river floods due to the northern snow melt-off.
For the channel cat, the bait must smell good to them. It is a safe bet that if the bait turns your stomach, the catfish will love it. You can also use cut bait, dough bait, whole minnows, cheese, chicken livers, night crawlers and commercially made stink baits, like Sonny’s.