The Philippines has two broad seasons: dry (roughly November to May) and wet (June to October). The dry season is when most travelers visit, and for good reason — days are sunny, seas are calm, and island-hopping is straightforward. January through March is particularly pleasant, with temperatures in the mid-to-upper 20s°C and low humidity in most of the country. April and May are dry but hot, pushing toward 38°C in the lowlands.
Typhoon season runs from June through November, with peak intensity in July, August, and September. Typhoons can be severe — they regularly cause flooding, landslides, and flight cancellations across Luzon and the Visayas. That said, the wet season is not without appeal: prices drop significantly, resorts are quiet, and many days between storms are perfectly sunny. The central Visayas — Cebu and Bohol in particular — are relatively sheltered and remain diveable through much of the rainy season.
Mindanao has a different rhythm: much of it receives rain year-round, with no clear dry season, though Davao City tends to be one of the most consistently sunny places in the country.
Divers should note that visibility and sea conditions are generally best from March through May, when the water is at its calmest. Whale shark season in Donsol runs from November through June; in Oslob, Cebu, the sharks are present year-round (though the practice of feeding them is controversial).
If you're timing a visit around festivals, January is excellent: Sinulog in Cebu and Ati-Atihan in Kalibo both fall in the third week of January. February brings Panagbenga (Flower Festival) in Baguio. Book accommodation well in advance for festival weeks.