Kenya has a warm equatorial climate all year round. The best time to visit is during the drier and cooler months from June to October. The "short rains" fall in November/December and "long rains" from March to May, when heavy downpours can make road travel difficult.
The annual wildebeest migration from the Serengeti into the Masai Mara occurs from August to October, although the exact timing depends on the rains.
The best time for diving is September/October and January/February, when visibility is at its best.