The nearest expiration date for Los Angeles CARB Diesel (OPIS) vs. NY Harbor ULSD Futures (May 2018) is May 31, 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 Los Angeles CARB Diesel (OPIS) vs. NY Harbor ULSD Futures (May 2018) before May 31, 2018.