The nearest expiration date for ULSD 10ppm Cargoes CIF NWE (Platts) Futures (May 2013) is May 31, 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 ULSD 10ppm Cargoes CIF NWE (Platts) Futures (May 2013) before May 31, 2013.