The nearest expiration date for ERCOT Houston 345 kV Hub 5 MW Peak Futures (Jun 2018) is Jul 3, 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 ERCOT Houston 345 kV Hub 5 MW Peak Futures (Jun 2018) before Jul 3, 2018.