The nearest expiration date for Gasoil 0.1% Cargoes CIF MED (Platts) vs. Low Sulphur Gasoil Futures (Jul 2018) is Aug 2, 2018.
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 Gasoil 0.1% Cargoes CIF MED (Platts) vs. Low Sulphur Gasoil Futures (Jul 2018) before Aug 2, 2018.