The best time to visit Phi Phi is November to April, when the northeast monsoon keeps the Andaman Sea calm and skies clear. Sea visibility for diving and snorkelling is at its best from January to March. The peak tourist period falls in December and January, when the islands are busiest and prices highest.
May to October brings the southwest monsoon, with rougher seas and some rain. Some guesthouses and restaurants close for the low season, boat services reduce, and diving at exposed sites becomes difficult. The islands are not impossible to visit in the low season — the rain is often intermittent, prices drop significantly, and crowds thin dramatically — but plan around the weather and check that services are running before you go.