The dry season from November to April is the most comfortable time to visit. Temperatures are warm rather than brutal, and you are unlikely to get rained out. The rainy season runs from June through October and can produce heavy downpours; the road in from the highway can become difficult after sustained rain.
If you want to see sea turtles nesting, plan a visit between September and January, though the height of the nesting activity is usually October and November. Arrive at the beach after dark and walk quietly — the turtles come ashore in the hours after midnight.
Christmas and Semana Santa (the week before Easter) bring a surge of Mexican domestic tourists, particularly students. For the quietest experience, avoid those windows.