The best time to visit Ostional is just before and during an "ARRIBADA". These occur, as a general rule but not always , at the start of the moon cycle's last quarter. The best months to see the biggest ARRIBADAS are between July and December.
After an undetermined waiting time, close to the moon's cycle last quarter, the "arribada" will begin. At first, a few hundred turtles will come out on the beach, followed by a steady stream of animals for the next three days.
"Arribadas" occur throughout the year, at least once a month and, in some months, may occur twice. However, the the months between June and December, offer the best opportunities since the number of nesting turtles may be in the range of thousands as opposed to hundreds during the dry season months. This tour is only offered when there is an "arribada" happening, therefore pick up times need to be flexible.