The nearest expiration date for PJM DPL Zone Peak Calendar-Month Day-Ahead LMP Futures (Sep 2017) is Oct 3, 2017.
The best time to sell a futures contract is when you've already made money on the investment, but still have plenty of time left before the expiration date. Thus, a good option would be to sell PJM DPL Zone Peak Calendar-Month Day-Ahead LMP Futures (Sep 2017) before Oct 3, 2017.