The nearest expiration date for RBOB Gasoline vs. NY Harbor ULSD Futures (Mar 2013) is Mar 29, 2013.
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 (Mar 2013) before Mar 29, 2013.