The dry season is typically from December to April, however it can vary slightly every year and within specific locations in Costa Rica with the rain stopping or starting outside of that general bracket of time. But basically for five to six months, there is zero to little rain. The further north you go, the more noticeable the affects of the lack of rainfall are. The province of Guanacaste is in the northwestern portion of the country and is the driest of all of the landscapes; so much so that is it home to the unique tropical dry forest ecosystem. You can expect dusty roads where the pavement ends, vibrant flowering trees like the Cortez armarillo (golden trumpet tree) to be in full bloom, cool mornings and evenings and blistering afternoons, cooler ocean temperatures, fewer mosquitos, and blue skies and starry nights day after day.
The dry season months are some of the best months to see humpback whales, usually with babies, traveling the Pacific coast. They usually stay relatively close to the coastline, so whale watching is very accessible. It is also during these months that the ginormous leatherback sea turtle nests in northern Guanacaste. In the tropical dry forest, the dry season is a great time to observe wildlife since the leaves of most trees have fallen and animals aren’t as easily hidden. Further south, the dry season months offer slightly less treacherous conditions (torrential downpours, mud, mosquitos, etc.) to explore the famous rainforests, mountainous areas, and cloud forests. December through April are the best months to visit Costa Rica if you are seeking a full on beach vacation, as every day offers the perfect beach day weather.
The conditions during the dry season are a bit harsh though. The sun is very strong and the air is very dry. It is absolutely imperative that you wear sunscreen (please only wear sunscreen that is natural and reef-friendly) every single time you venture outdoors. It is also a good idea to bring a hat, lightweight long sleeve shirts, good quality sunglasses, and a re-useable water bottle that you keep filled and with you at all times. It is very important to stay hydrated since heat exhaustion and severe dehydration are a threat to your health and happiness while in Costa Rica. While the mosquitos are way less abundant during these months, it is still important to wear insect repellent, as dengue and other mosquito-borne illnesses can be contracted any time of the year.