Hi all, 
I've just joined this site with the possibility of doing a trip to Kenya. 
I have been once before (although not through this site) and that was in October to catch the end of the migration, and the weather was quite warm. 
I was thinking of trying a different time of year (May) but after a little research found out that April/May are the two wettest months which makes wildlife harder to spot and brings the mozzies out in force. 
Jan/Feb are hottest and makes animals easier to spot coz they hang around the water holes.  
Jun - Oct is best for migration. 
Sorry if this rains on your parade (literally) but hope it's been a help.