The nearest expiration date for Los Angeles Jet (OPIS) vs. NY Harbor ULSD Futures (Apr 2018) is Apr 30, 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 Jet (OPIS) vs. NY Harbor ULSD Futures (Apr 2018) before Apr 30, 2018.