Tortuguero is wet all year, even by Costa Rican standards. Pack for rain whenever you come. The green turtle nesting season, roughly July to October, is the classic time to visit, with night walks run under strict rules to protect the turtles.
Wildlife viewing in the canals is good year-round, especially early in the morning. February to April can bring somewhat drier weather, but this is rainforest and the forecast should never be treated as a promise.