As we said, Costa Rica is in the unique position of having a couple different oceans to choose from when picking your surfing destination. For that reason, it also presents an almost ideal surfing spot year-round.
The Northern Pacific coast is going to look ideal when the doldrums of winter set in for those of us north of the equator (keep in mind that December to April is summer in Costa Rica). From December to March, it has offshore winds that provide a nice swell. The Caribbean Coast is also going to be a good bet from November to March. Count on the Central and Southern Pacific coasts from May to November [source: SurfCostaRica.com]. Of course, keep in mind that these are just guidelines, and weather patterns vary and change.
You also might want to take into account crowds. If you're a northerner and you think escaping the snow and sleet is a great idea in December and January, you're not alone. While it's the hottest and driest time of year in Costa Rica, it's also the busiest for sun-deprived tourists. Take your love -- or hatred -- of crowds into account when you're planning your trip.