Night is generally the best time to fish for cats, but when the water is cooler they sometimes bite just as good (if not better) during the day. As for bait, there are two general schools of thought...many people say get the nastiest, stinkiest, rotten bait you can find, but most of the best catfishermen will tell you the fresher the bait, the better. This is especially true for bigger cats. I like to use shad, bluegill, or yellow perch (use whatever the cats are used to seeing in their home lake...I've even heard its best to gather bait from the same waterway you will be fishing). For channels, use cut pieces of the fish. For flatheads, you want live bait. Blues seem to like it either way.
Rig up your bait with a big circle hook on a slip sinker rig. Sometimes a bobber set up can be good as well. They will likely be fairly shallow this time of year...sometimes in as little as 1'-2' of water. There are a couple ponds I fish where I commonly spot channels cruising around 1'-3' of (clear) water, usually near sunfish.
I am by no means an expert, and some people may disagree with me, but this is what I've found actually works (for me). A lot of people say fishing in Colorado is much different than other states, so it could be different for you.