Summer in Costa Rica runs from December to April and during this time it is very hot and dry. This is the high season tourist wise so if you are thinking of travelling here at this time of year be prepared to pay extra for accommodation, flights, car rental.
For the rest of the year it is pretty much rainy season and while still hot can be pretty wet! Avoid September, October and November when the rain really falls and travel can be positively inconvenient.
Costa Rica is becoming much more mainstream as a travel destination and with both the Caribbean coastline and the Pacific coastline offering good surf throughout the year it is becoming a popular surfing destination.