An important factor in deciding when to exercise is where you exercise. If you need a facility such as a gym, swimming pool, or exercise equipment, you probably want it convenient to either home or work. That way it can be a place to stop on your way to or from work or a place to walk to during the lunch hour. If you choose cross-country skiing, you'll have to wait for winter's snows. So find another activity for the other seasons, such as walking or playing soccer or softball.
Some people find they exercise more regularly when they share the task with another person.
Your workout buddy can be a spouse, friend, child, or someone you met at the locker room. If your workout is a competitive sport such as racquetball, be sure to book the court as you leave so you and your buddy can schedule the next game. If you are an exercise loner and walking or jogging is your game, you need little equipment so you can do it any time and just about any place. But commit to a time and place. Another key is keeping a record of your activity.