Quote from jwcapital:
For September and the other quarterlies, the options expire on Friday at the open. A special opening quote is published and all ES futures and options are settled based on that special opening quote. The last day of trading the ES futures is Thursday at 4:15PM. A lot of things can happen between 4:15PM and Friday's special opening quote. When I have any long option positions, I exit them on the Friday before expiration or the following Monday--the latest. If I am short naked options, and they are far OTM, I let them expire. If it is close, I look to close them before Thursday's close. Even though the Sep ES is thinly traded, it does keep up with the S&P index. If I trade any same-month spreads (combos of long and short options), I exit them either the Friday before expiration or the following Monday. For serial expirations (where ES FUT and FOP expire on Friday at 4:15PM and are settled based on the close of the ES future), I basically follow the same rules.