Even though Costa Rica is in the northern hemisphere, the names of the seasons are reversed and 'summer' starts late in November. This is not because of a large temperature difference but because it tends to be drier from December through April. Note—drier, not dry. If you visit the rainforest, you should plan on getting wet.
The high season
The high season in Costa Rica runs from December through April. The dry season is a month or two shorter along the southern Pacific coast. July also tends to be a dry month on the Pacific slope.
The main attraction of traveling in the high season is the weather. It is less likely that it will rain on you, and in some areas (notably Guanacaste) the wildlife is concentrated near shrinking water sources and easier to spot. Obviously, another motivation for North American and European travelers is escaping the snow and cold at home. Not only is Costa Rica warm, but travelers from the north will be rewarded with an extra few hours of sunshine every day—at this latitude the days are longer.
The main problem with traveling to Costa Rica during the northern hemisphere winter is that it is the high season. It is harder to get reservations, everything is relatively crowded and the prices are higher.
From Christmas until the end of the first week in January, and Easter week are doubly busy periods with lots of international travelers, and many Ticos traveling (the beaches are especially crowded).
The green season
The rainy season, or winter in Costa Rica (May through November). Welcome rains during the rest of the year bring about a general greening and freshen the countryside. Rains usually come in afternoon thunder-storms, leaving; the mornings sunny and the night sky filled with stars.
This period is called "invierno" (winter or rainy season) or "temporada verde" (green season). Rainfall on the Caribbean slope is more evenly distributed throughout the year, with marked dry periods from May through June and again from September through October. If you are thinking about a trip to Costa Rica during the green season, don't let the weather stop you.
If you’re looking for cheaper accommodation then this is the best time to visit except during the rainiest months, September and October, when some hotels shut down.