Costa Rica has two seasons, and one detail almost nobody accounts for: the Caribbean does not follow the same calendar as the Pacific. Planning without knowing this is the most common mistake.
Dry season — December to April
This is high season. The Pacific and the central valley get almost no rain, days are sunny and dirt roads are in good condition. It is also when prices rise, parks fill and accommodation needs booking ahead, especially over Christmas, Easter and the first two weeks of January.
Green season — May to November
Calling it the 'rainy season' overstates what happens across most of the country. The usual pattern is clear mornings and afternoon downpours, short and heavy. In exchange the landscape is spectacularly green, prices drop noticeably, parks are quiet and wildlife is more active.
September and October are the wettest months on the Pacific, with rain that can last for days — but they are precisely the best months on the Caribbean, which enters its driest period then. If you travel at that time, flip your itinerary to the other coast and the trip works.
Wildlife calendar
Humpback whales appear in two windows: December to April for northern populations and July to October for southern ones, peaking in August and September. Olive ridley arribadas at Ostional run July to December; green turtles nest at Tortuguero July to October; leatherbacks on the northern Pacific October to March. The quetzal is easiest February to July.
Choosing by priority
For beach and guaranteed sun, January to March. For wildlife and lower prices, May, June or November. For big surf, June to September. For the Caribbean, September, October or the February-March block.
A warning about roads
In the green season some unpaved secondary roads become difficult, and river fords — crossings without bridges, very common on the Nicoya peninsula and on Osa — can rise within hours. If you rent a car in those months, take a vehicle with genuine clearance and four-wheel drive, and never cross a ford when the water is high.
Easter week
Worth noting separately: on Holy Thursday and Good Friday the country stops, many businesses close, alcohol sales are restricted in some districts and Costa Ricans head en masse to the coast. It is the most expensive week of the year and the busiest on the roads.