In Costa Rica, Easter is a huge holiday and is celebrated for a week. It’s called Semana Santa or Holly Week and in 2016 it will span from March 20th (Palm Sunday) to March 27th (Easter Sunday). During this time many Costa Ricans take vacation time so the beach towns are at their busiest and hotels are booked up months in advance. There will also be lots of traffic heading to the beach at the beginning (Friday and Sat March 18th-19th) and at the end (Sunday or Monday the 27th-28th. During the mad rush back to San Jose, sometimes Highway 27 is turned into a one way highway going from the Pacific coast back to the city. If you are trying to come the opposite direction at that time, you might have to take slower back roads.
As for religious activities, you could ask your hotel what the schedule of services is at the local Catholic church nearby. There also may be some parades if you are staying near the city so be sure to ask about those too.