The dry season runs from May to October — the best time to travel. Roads are passable, the landscape turns to golden savannah, and the coast is clear for surfing. The wet season (November to April) brings heavy rain, particularly in West Sumba, which can turn dirt roads to mud.
The exception is the Pasola festival, held in February or March depending on the lunar calendar and the appearance of sea worms (nyale) off the coast. Dates are set by Marapu priests and announced late — serious travellers contact local guides or check with guesthouses in Waikabubak a week or two before the likely window. Pasola takes place at multiple sites across West Sumba on different days; Lamboya and Kodi are the main venues. The atmosphere is intense and the ceremony deeply meaningful to the Sumbanese — behave accordingly.
Easter week brings related ceremonies and horseback processions in some western villages. If you are on the island during either period, nothing else will compete.