There is one calendar that matters on Pentecost: the Naghol season. The land dive is performed every Saturday from early April through to the first or second weekend of June, timed to the yam harvest and the elasticity of the chosen vines. If you have come for the dive, this is your window — and the southern part of the island fills up, so book flights and accommodation well in advance.
Outside the dive season, conditions are quieter and the practicalities a little easier. The dry season (May to October) is the more comfortable time to be on the island; the wet season from December through March brings heavy rain, mud on the tracks, and the risk of cyclones, though daily life carries on. Avoid the worst of the cyclone weeks in January and February if you can.