Whenever I take a trip overseas, i try to do a bit of research before hand, so if i may suggest, google costa rica and you should get a fair idea of what time to visit the place. And my experiences with utilising
http://www.tripadvisor.com/ has been nothing but great during my trip to spain. With these guys you can literally organize your trip from top to bottom even way before leaving for it. So it may be a good idea to check them out as well. have fun and in off season time, you are bound to get great discounts, anywhere in the world.