Costa Rica: When to Go
The high (summer) season runs from November to April, and though it's also classed as the 'dry' season it doesn't necessarily mean there will be no rain - but there should be less of it! Prices are higher during this time. The 'green' season (from May to November) can still be a good time to visit, especially the early months when days can start off with sunshine then turn rainy in the afternoon/evening. More careful planning is required then, though, as some roads become impassable (even in a 4x4).
Bird-watchers will be happy year-round: flocks migrate to Costa Rica between September and November, though those in search of the resplendent quetzal will be more sucessful between November and May. For turtle enthusiasts the leatherback can be seen nesting in Tortuguero (on the Caribbean coast) from mid-February to mid-April, or on the Pacific coast between September and March. Green turtles nest on the Caribbean coast during August and September.
There are public holidays throughout the year, normally around one per month. The main celebration is Semana Santa (Holy Week or Easter) - hotels are busier during this time so availability may be scarce.