Lake Naivasha is a destination that you can come and visit all year round. The most ideal time, however, is between June and October. During those months it is the summer and it will be hot, but still the ideal time to visit.
You can also visit between November and March, which is the winter season which is cooler, but there might be some rain.
April and May are not advised because that is the time when the area gets the most rain, and most likely all activities will be cancelled. However, you can visit whenever you want and experience the beauty of the area. The best time to visit is when you are free and have the time. Don't worry about the weather unless you are dying to get out and visit the parks. No matter what, make sure that you dress accordingly so that you don't end up falling ill.