Coca is a tiny little town in between Cuéllar and Segovia...it's actually about an hour away from Segovia (northwest) on, if I remember right, SG-342. We went there in May. It's a nice detour drive, and at one point is nothing but flat and straight...a good chance to see how fast you can get our rental car going...we hit 170 kph and loved it!
Coca has grown over the past ten years, and there are a few places to eat, but it still seems to be one of those towns where people own a piso for weekend use. As I mentioned, though, even between 1999 and May of 2002 I'd noticed considerable change in the town.
To answer your fireworks question: The fiestas they have there are the Santa Cruz fiestas and are held in early May. I don't know of any in August, but you can always check their website for more information:
Coca City Hall The site has all the info you'll need on dining, lodging, etc. Enjoy the side trip!