The nearest expiration date for ULSD 10ppm Cargoes CIF NWE (Platts) vs. Low Sulphur Gasoil Futures (Oct 2015) is Nov 3, 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 ULSD 10ppm Cargoes CIF NWE (Platts) vs. Low Sulphur Gasoil Futures (Oct 2015) before Nov 3, 2015.