Periodization is a planned approach to competition that is based upon balance. Their training regimen is designed to allow for peak performance at these predetermined times. Combining off court training, cross training and increasingly challenging competition, periodization is, in my mind, the most balanced approach to training. The summer may be the time of year when you can play tennis most frequently. You probably are competing but reasonably.
Summer is the time to learn and develop.
In the fall, you will hit frequently, but compete infrequently. You will probably increase your strength training and begin a cross training aerobic plan. Winter months are your rest period. You will hit two or three times per week, but you won't compete. You'll be strength training and really working on your speed and endurance.