For my opinion the the dry season from December to April. Although you might see the Olive Ridley Turtles at the rainy season and reptiles and amphibians are more abundant.
The Hacienda Baru takes care of a nature protected area, own beach and wildlife. The effort of the owner is marvelous. Birding is outstanding; each morning and evening we had two chestnut-mandibled toucans close to our house. The meals are fresh prepared and delicious!
Baby turtles are hatching from September to November so nesting season is 45 to 60 days before.
More information: http://www.haciendabaru.com/