Kenya has four seasons, though the concept translates differently than in Europe or North America. The two rains — the long rains from March to May and the short rains in November — divide the year into wet and dry periods.
July to October is the prime safari season. This is the dry season, when vegetation thins out and wildlife concentrates around water sources, making game easier to spot. It also coincides with the Great Migration: from around July, some 1.5 million wildebeest, zebras, and gazelles cross from the Serengeti into the Maasai Mara, pursued by lions and crocodiles waiting at the Mara River crossings. This is spectacular and justifiably famous. It is also crowded — the Mara fills up with vehicles in August and September. Book camps well in advance.
January to March is a second excellent period, particularly for the Amboseli and other southern parks. The vegetation is drier, Mt Kilimanjaro views across the border are clearer, and there are far fewer tourists. Coastal beaches are at their best in this window too, with hot, dry weather and calm seas.
The long rains (March–May) bring lush, green landscapes and low prices, but some lodges close and dirt roads become impassable. Birding is exceptional. If you don't mind occasional downpours and want to avoid crowds, this can be a rewarding time.
November to December sees short rains — generally afternoon showers rather than all-day grey. Prices drop, parks are quiet, and the bush is green and photogenic. Not a bad time at all, and often overlooked.
For beach holidays on the coast, the most reliable weather is December to March and July to August. The main rainy season (April–May) sees the coast at its least appealing — hot, humid, and wet.
The Kenyan highlands, including Nairobi, are mild year-round. Evenings can be cool even in the dry season; bring a jacket.