When thinking of the best time to visit Kenya, here are three things that you need to take into account.
Obviously, a safari would not be complete if you do not see any animals. For instance, between July and November, animals in Serengeti (Tanzania ) migrate to the Masai Mara in Kenya and then back again to Tanzania. There is also a flamingo migration into Lake Nakuru and Lake Bogoria. This would mean that these are good times to visit these parks.
Kenya nonetheless has many parks and the animal migration patterns are different. It is definitely advisable to check with your tour advisor before you travel.
June and July are the cold months in Kenya. Mind you it only deeps to about 12degrees centigrade. So it may not be as cold as it sounds. The trouble is the rainy season.
Rains cause some roads to become impassable. For example, there is a stretch of road about 15km to Lake Baringo that has been washed off by flush floods originating from nearby hills. If you are booked to travel to this place in the rainy season, you may either get stuck there for a period of time or become unable to reach there altogether.
If you are budget cautious, then you may want to travel when it is low season. This tends to be between April and June or October and December. At this time, lodges, hotels and flights tend to be much cheaper. However, the disadvantage of travelling then is that you may not see much game, especially the great migration.