Generally, for sails our crew likes a couple days after the Full and New moon with the full being better. I think in Dec. you will be fine. Pretty much all of Costa Rica starts to have great fishing from Thanksgiving to April 1st. after the 1st of April most of Costa Rica slows, were we pretty much still have good fishing. It is just more marlin and tuna, farther offshore and less sails. Good luck! If you ever down our way drop us a line...