Zanzibar has two rainy seasons: the long rains (masika) from mid-March to late May, and the short rains (vuli) from October to December. The best time to visit is during the dry seasons: June to October and January to mid-March. The peak months of July and August bring reliable sunshine and calm seas ideal for snorkelling and diving, as well as the Zanzibar International Film Festival (ZIFF) in July.
During the long rains, some beach roads become impassable and dhow trips may be cancelled, though prices drop sharply and the island is far less crowded. The short rains are typically lighter and more intermittent than the long rains — many travellers visit in November and December with no problems.
Water temperature stays around 24–28°C year-round, making swimming pleasant at any season. Visibility for diving and snorkelling around Mnemba Atoll is typically best from October to February, when the sea is calmer on the northeast coast.
The Sauti za Busara music festival, held in Stone Town in February, is one of the best reasons to time a visit — three days of African music, dance, and theatre in the fort and gardens of the old city.
Zanzibar's equatorial position means the difference between seasons is less dramatic than in southern Africa. The real question is not whether to go but whether you want to avoid the crowds of peak season or the inconvenience of heavy rain.