The nearest expiration date for RBOB Gasoline vs. NY Harbor ULSD Futures (Jul 2015) is Jul 31, 2015.
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 RBOB Gasoline vs. NY Harbor ULSD Futures (Jul 2015) before Jul 31, 2015.