Winter (June - September): Like mainland Tanzania, the best time to go to Zanzibar, Mafia or Pemba is during the dry season from June to September. Due to its close proximity to the equator, the temperatures vary very little through the year, with maximum daytime temperatures hovering between 25°C / 75°F and 28°C / 82°F throughout the year. The dry season is famous for its clear blue skies and sunny days. This is also the best time to go scuba diving at the majority of sites as the water will be crystal clear and visibility will be at its peak though days are occasionally windy and choppy (especially in July when the “kusi” wind is at its peak - excellent news for kite surfers.
Going in June or beginning of July will have the added bonus of the vegetation being green and lush after the rains. September is usually spice harvest season, and it is quite fascinating to watch the farmers harvest cloves by hand. Winter has the added bonus of combining beautifully with a safari to Ngorongoro Crater, Serengeti and Mount Kilimanjaro.
Summer (January - March): January to March is summer and our second favourite time to visit Zanzibar, Mafia or Pemba. The summer months bring with them beautiful dry, sunny and warm days with bright blue skies very similar to winter, with temperatures averaging around 28°C or 82°F. The north-eastern “kaskazi" wind is at its peak during January, affecting the north and east coasts of the island, but starts to mellow as March approaches.
Spring (November - December): Spring is the second of the island’s two rainy seasons and is known as the time of "short rains”, usually in the form of late afternoon showers. The rains are intermittent and not too much of a bother, but we do feel that when choosing to go to a beach destination, dry season would be better, making the period of November to December not our top pick.
Autumn (April - May): Autumn is the least desirable tome to visit any of the islands, be it Zanzibar, Mafia or Pemba. April and May are monsoon season with spectacular storms and is known as the season of “long rains”. The waters are rough and murky, and the beaches wet and windswept. Many hotels close altogether during these months to refurbish and upgrade their lodgings for the next season. Luckily these long rains coincide with mainland Tanzania as well, which rules out only these two months, and leaving 10 months for a magical Bush + Beach combination. Surprisingly, this season is a good time to go diving specifically in the north of Zanzibar because of its wind free days, underwater visibility is great and you have a chance to see more and bigger fish on most dives than the rest of the year in this part of the island.