The nearest expiration date for Singapore Gasoil (Platts) vs. Low Sulphur Gasoil Futures (Jul 2014) is Jul 31, 2014.
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 Singapore Gasoil (Platts) vs. Low Sulphur Gasoil Futures (Jul 2014) before Jul 31, 2014.