Costa Rica has a more complicated weather pattern to that of its neighbouring countries. The Caribbean and Pacific coasts have different seasons as does the central part of the country.
San Jose, the capital, the main areas inland and the northern pacific coast all have similar weather patterns. For these regions the dry season is from December to April and the wet from May to October. November can also be wet but will generally receive about half the rain that you can expect in October.
The northern Caribbean coast around the area of Tortuguero National park and the Osa peninsula in the south west are wet all year round, but they also have some of the best wildlife viewing opportunities in the country. The southern Caribbean coast is best visited in September and October, when it is driest