That depends on you. The high tourist season known as the “high season” runs from December through April which is Costa Rica’s summer so there is little to no rain and a lot of sunshine. This is also when the beaches are crowded, everything from lodging, food, and airfare is more expensive. But it’s also when things are a bit livelier especially at the beaches. During this time you should make reservations 3-6 months in advanced and at least 12 months during Christmas, New Years, and Semana Santa (Holy Week). It’s popular because of the weather especially when it’s cold and snowing in places like the east and the midwest of the United States.
The low season is May through November there still plenty of sunshine and warm weather but it does rain starting in mid-afternoon and the evening. The rains produce lush tropical gardens which is very beautiful. The low Season typically features sunny mornings followed by rain showers in the late afternoon.
We prefer the low season since the beaches are not crowded, you don’t need to make advanced reservations (although it’s recommended), and prices are 20-50% less than during the high season. In our opinion the best time to visit is during mid-July, a break in the rainy Season, called “El Veranillo,” occurs, and the region experiences a few weeks of dry conditions. So you get to enjoy summer type weather at winter prices.