If you are staying at a resort, you can contact them and they can arrange transportation for you. It will cost extra. If you are not staying at a resort, your cheapest route is to take a taxi, official taxi, from the airport to the bus terminal, from there take a bus to Jaco. If you are arriving late, you will have to stay in San Jose overnight and get a bus the following morning. Buses don't travel late at night. Buses are very cheap to take. But I am not even sure if the buses run on Christmas day here.

i live there. there's tons of red taxis everywhere...but you can also take an orange cab from the airport.