The nearest expiration date for Mars (Argus) vs. WTI Trade Month Futures (Jun 2017) is Jun 2, 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 Mars (Argus) vs. WTI Trade Month Futures (Jun 2017) before Jun 2, 2017.