Zanzibar has two rainy seasons and two dry periods — but that doesn't tell the whole story. Because rain on Zanzibar is different from rain in Tanzanian cities: it often rains heavily but briefly, and the sun appears again quickly after. A rainy day on Zanzibar is rarely a lost day.
January–February: excellent
This is the best combination of good weather and low occupancy. Temperatures are pleasant (25–30°C), rain is minimal and beaches are not crowded. Ideal if you seek peace and quiet. Downside: fish and coral activity is less spectacular than in peak season.
March–May: avoid for beach holidays
The "long rains" hit Zanzibar hardest in these months. It rains for days at a stretch — and that is different from the brief showers of other months. Many hotels close in April. If you do go in this period: prices are low and Stone Town is relatively quiet.
June–October: the absolute highlight
This is the dry season on Zanzibar AND the safari season on the mainland — and the perfect combination. Diving and snorkelling are at their finest (visibility 20–30 metres), the wind is pleasant and temperatures are ideal (24–28°C). Occupancy is high in July and August — book early.
November: short rains — but still fine
The short rains in November are more intense than the name suggests, but the periods are shorter. Typically: morning sun, afternoon shower, evening sun. For many travellers this is perfectly acceptable. Prices are lower than peak season.
December: the second peak
Around Christmas and New Year Zanzibar is popular with European and Australian travellers. Prices rise, occupancy is high. Weather is generally good — the short rains are then already coming to an end.
My practical advice
Combining Zanzibar with a Tanzania safari? Plan your safari in July–October and your Zanzibar week immediately after. It matches perfectly: dry safari season, followed by beach weather on the island.
Zanzibar only? June or October are my favourites: good weather, lower occupancy than the busier summer, and excellent diving conditions.
Avoid April if you seek sun and rest — unless budget is your primary consideration and you're willing to tempt fate.