Rest and Recovery--
In triathlon, since you are training for 3 sports and including stretching and strength regimens, you need to have an organized training plan or you will end up doing too much and overtraining, resulting in burnout and/or injury. One way to prevent this is to include a) 1 day off per week b) 1 R&R week a month and c) an off-season where the training is less vigorous and less formal.
In your R&R week, reduce the volume by about half and lower the intensity. Make sure you have fun. Keep it simple and loosely structured. Do something different--play tennis, basketball, or soccer. Go roller-blading. Be active but challenge your body in different ways to keep yourself fresh. When you return to your training plan in 4-5 days or so, you will be revitalized, completely recovered, and stronger than before.