Zanzibar is a good place to visit all year round. Even in the rainy season the island is a great place. It's not raining 24-7, the island is quiet and great for diving for example. Check the activitites you can do outside of kitesurfing here.
For kitesurfing the best time of year is all year round except from April and November. The reef protects the inside waters, so the lagoon inside the reef is flat, shallow knee-deep water, perfect for beginners and experts.
Check when the Kite Centre is opened
Wind
The best months for beginners are from mid-June until mid-October and the end of December until mid-March. The wind is always side onshore. From June until October the Kusi blows from the south-east, in December the Kaskasi blows from the north-east.
The best months for experts are from mid-June until mid-September and the end of December until the end of February.  While the above listed time-frames are the norm, variations do occur. Therefore we do recommend you to check the wind history on the island.
The average wind speed is around 16 knots, usually increasing towards the end of the day in the late afternoon and early evening. The most commonly used kite sizes are 12, 10 and 9 meters, with some occasional 7 meter kiting, depending on your body weight and the wind speed.
More information about the kite conditions