The nearest expiration date for PJM AECO Zone Off-Peak Calendar-Month Day-Ahead LMP Futures (Sep 2018) is Sep 28, 2018.
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 AECO Zone Off-Peak Calendar-Month Day-Ahead LMP Futures (Sep 2018) before Sep 28, 2018.