Yap's single biggest draw — manta ray diving — is available year-round, which makes it unusual among the world's great dive destinations. The reef manta rays that gather in Mi'l Channel are resident, not migratory, and encounter rates remain high in every month. If mantas are your primary reason for coming, you can go when flights are cheapest or when your schedule allows, without worrying about seasons.
For the best diving conditions overall, the dry season from November to May is preferable. Visibility is typically better, winds are lighter, and the surface conditions are calmer for boat trips. The rainy season runs roughly from June through September, when the Inter-Tropical Convergence Zone brings heavier precipitation and occasionally rough seas. Rain on Yap tends to come in bursts rather than sustained downpours, and you can still have perfectly good diving days during the wet season — but your chances of consistently clear water are higher in the dry months. Water temperatures stay warm year-round, hovering between 27 and 30 degrees Celsius, so a 3mm wetsuit is sufficient for most divers. Traditional ceremonies, including the mitmit dances that are among the most impressive in Micronesia, occur throughout the year and are tied to the village calendar rather than to tourist schedules; ask locally when you arrive about upcoming events.