The best time to prune witl always be in the Fall/Winter. That being said most people call for pruning in the Spring/Summer when the foliage is out. Trees have expended lots of energy producing this foliage and on new growth. It takes energy to heal cuts made by arborists. Energy they don't always have to expend in warmer months. Warmer months are also when insects are most active. They are attracted to the fresh cuts and sometimes carry diseases. If pruning is done in these months it should be light and for specific purpose. Also iof large limbs are to be removed it should be done in multi phased attack.