The nearest expiration date for Brent Crude Oil vs. Dubai Crude Oil (Platts) Futures (Apr 2017) is May 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 Brent Crude Oil vs. Dubai Crude Oil (Platts) Futures (Apr 2017) before May 2, 2017.