The dry season, from mid-December to mid-April, is by some distance the best time. Sunny days, light winds, calm seas for the ferry crossing, and the brown pelican nesting season on the back side of the island is in full swing. This is also when day-trippers from Panama City arrive in greatest numbers — weekends can feel busy on Playa Restinga, weekdays much less so.
The wet season runs May to November. It rarely rains all day, but afternoon downpours are routine and the seas can be rough. The Virgen del Carmen festival in mid-July is the biggest event on the island calendar, with a boat procession and fireworks; worth timing a visit around if you can.