You can catch crappie on Chickamauga 365 days a year! You simply have to adjust your locations and techniques to what the fish are doing.
That said, MY favorite time is from right now until the first week of April - the period known as "pre-spawn." That's when long-line trolling works best. It requires a little boat set-up but once done, it is super easy even for kids and inexperienced anglers, super relaxing and produces LOTS of fish. Granted, it may not as much fun as "single-poling," casting for crappie, but for the aforementioned reasons, it is HIGHLY effective. This video is an excellent examply of the reason I like it.