The best moment to come to Costa Rica really depends on you. Costa Rica has two seasons: the winter, also called green, rainy or touristic season and the summer, also called the low or dry season. Note that those seasons are reversed relative to North American countries. Located in the South Pacific, the temperature is pretty much the same all year, around 80 ºF (27 ºC).
Most people travel during our "tourist season" (December to April) to head south and escape the winter of their home country. Other reasons consist of the splendid sunsets, the best chances of spotting rare wildlife species and tons of shooting stars. In comparison to many other tourist regions of Costa Rica, the beaches, national parks and rainforest trails never get crowded in the South Pacific.
On the other hand, there are plenty of advantages to visit the South Pacific coast from May to November. At this time of the year you can find discounts on flights, hotels and activities. Fishing is as good as high season and sometimes even better. Try some delicious tropical fruits you’ve never seen before and smell colorful flowers blooming everywhere. It is also the perfect time to appreciate a good massage under powerful waterfalls, take marvelous pictures on the green paths and experience driving a 4x4 car through the gravel roads crossing creeks and mud. Mountain hiking trails may be a bit slippery, but not enough to stop an adventurer. It usually rains in the late afternoon though much less than the last decades. Most of the time, the rain lasts a couple of hours only, just enough to cool down the temperatures.