Depending on your goals - sunrise yoga, whale- or bird-spotting, hiking deep in the green jungle, or a mix of doing nothing and doing everything - the best time to visit might vary with weather, migration and nesting patterns, as well your local school calendars.
1. Look at the Patterns
Costa Rica has some distinct travel seasons, largely driven by the rain:
The dry season runs from mid-December through April, with just a few scattered showers per month. It’s perfect timing for Northern Hemisphere travelers on school break-- in fact, March and April are normally the driest months of the year (but they are also the hottest and busiest.)
The green season starts beginning in May, and runs through November. Much of the early part of the season - from May to July- it will usually rain a few times a week, but you will usually have sunny days outside until the afternoon rains. By late September the wet weather starts to ramp up, with the rain increasing steadily through early December. We are closed from mid September through mid November as roads and paths can become more difficult and rivers impossible to cross.
One last note: Weather in the Equatorial Pacific can be impacted by global weather patterns - for example, El Niño, which generally decreases rains on the Pacific side of the country, and La Niña, which may cause increases. Many government agencies issue annual predictions for these patterns, including NOAA in the United States.
Migration patterns have the most impact on the birds and marine life you will see in the Osa. Bird activity - which is always high - gains momentum during the rainy season. Migratory birds from North America - including the Baltimore Oriole - leave their breeding grounds in the North in late summer, and most have arrived in Costa Rica by October.
There are Humpback whales nearly year round, but it's really two different groups - one from near Chile, one along the upper Pacific Coast of California. Each moves to our warmer waters during the winter in their respective hemispheres, with a short gap between them in May and June. The very best time for Whales is May through October.
Dolphins are year-round in the Golfo Dulce. January through March sees Whale sharks entering the Gulf.
To see nesting turtles, the right time is the few days around the new moon (any time after the third quarter and before first quarter moon), during high tide. Our part of the peninsula hosts both Olive Ridley and Green sea turtles. There is a growing population of Hawksbill turtles as well - they nest more to the north of the Golfo Dulce, and seem to use the mangroves near Ojo del Mar as a nursery habitat.
2. Check the Calendar.
Once you have settled on the optimal time for your visit, start looking at schedules - your own, transportation, and other things that impact your travel and lodging reservations.
Book early. Many experienced eco-tourists start planning as far as 14-18 months from their intended stay; most book their primary air tickets and lodging about a year out - and further if visiting during a busy time of year.
Leave enough time. Travel takes a bit longer in this part of the country. A few things might impact the time you allow for your visit:
The flight between Puerto Jimenez and San Jose leaves many times a day; check flight schedules to determine the number of nights you will stay at Ojo del Mar.
Similarly, ground and boat travel to parks, waterfalls and spotting destinations - while an integral part of the activity - can limit the number of things you can fit in on a given day. Spend time on your "wishlist" of things to see and do, so you can see how they fit together in an itinerary. You might find you want to add more dates than originally expected.
Finally: don't rush yourself. You'll want to be able to linger and take in the rainforest, or have time to just lounge by the pool. Expect a slower pace than you might have in other parts of the world.
3. Get your Documents in Order
Once you know know your calendar - the time of year, your basic itinerary and the number of days you'll be away - you'll have what you need to make reservations, and set up appointments.
Talk to your Doctors. Depending on your other destinations and your overall health, your doctor might recommend a yellow-fever inoculation, diphtheria/tetanus, typhoid, and hepatitis A vaccines. If you are entering Costa Rica from some South American countries, you will need proof of your immunizations at the border. The risk of malaria is low—but all these are worth discussing with your doctor, especially for travelers under 13 and over 65, or anyone with health concerns - especially those exacerbated by heat, humidity or activity.
Check the status of your passport and other travel documents, as well whether you will need a visa by checking with the Costa Rican Embassy in your country.
Then, start the fun part! Spend some time researching and planning your packing by watching documentaries, researching on the Internet, and following hashtags for the #osapeninsula. And, when you are ready to start packing, check out our packing guides.
Check out our Maps and Apps post for Weather, wildlife-spotting and other tools.
Interested in tracking migrations? The project Journey North is written to involve young people in observing and helping map the waves of migrations that move between the North America and Central America. Other sites, like Ocearch and National Geographic track the movement of marine life through the Pacific and Golfo Dulce using satellite transponders.