The Bluegrass Festival that John referred to is near Clinton, but it's not associated with the Katy Trail in any way, and it's actually about 5 miles from the end of the trail in Clinton. I listed it in the events section of this website just for anyone who might want to add something a little different to their Katy Trail experience.
I've had a very hard time finding information on this festival - it's a low-tech operation and they have no website. If you call the number listed for
the event, they might be able to give you a little more information.
Meanwhile on the eastern end of the trail, check out the St Charles Bluegrass festival that Rhona mentions above.