Quote from PocketChange:
ES Settlement calculations vary by contract:
CME uses 3 sets of calculations:
SOQ For Quarterlies
EOD: Serial Months between quarterlies.
ESF: For End of Month and Weeklies
ES American Style QUARTERLY OPTIONS are exercised into expiring cash-settled futures, which settle to the SOQ calculated the morning of the 3rd Friday of the contract month. In-the-money SERIAL OPTIONS are exercised into futures at their strike prices at 3:15 p.m. on the 3rd Friday of the contract month. (Settlement by SOQ or 3:15 Daily Settlement procedure)
http://www.cmegroup.com/trading/equity-index/settlement.html
http://www.cmegroup.com/education/files/Understanding-the-SOQ-2011-11-04.pdf
EW End of Month and Weeklies European Style. On expiration day, all in-the-money options as of 3:00 p.m. CT will be automatically exercised.
"CME Group calculates and disseminates special fixing prices that are used to determine whether end-of-month and weekly options on Equity Index futures are in-the-money at expiration. We calculate these special fixing prices in the last 30 seconds of trading before the 3:00 p.m. (2:59:30-3:00:00 Chicago Time / CT) options expiration. Then, we immediately disseminate the prices under the ticker symbol ESF for S&P 500 options, and NQF for Nasdaq-100 options."
http://www.cmegroup.com/trading/equity-index/SandP-EOM-options-fixing-price.html
These special ESF settlement prices are used every Friday except the 3rd Friday of each month and on the last business day of each month: Approx 52 times a year BUT 4 times a year the Quarterly contracts closes and settles early at 8:30am using the SOQ calculation and the 8 serial months use EOD settlement.
It gets a bit grey cause they publish a daily settlement procedure that is used for early exercise and assignment plus the 8 serial months between quarlerlies: http://www.cmegroup.com/market-data/files/CME_Group_Settlement_Procedures.pdf
http://www.cmegroup.com/market-data/settlements/settlements-details.html
The option specs are worth reading... http://www.cmegroup.com/trading/equity-index/us-index/e-mini-sandp500_contractSpecs_options.html
If you think this is confusing read upon the currency future options: weeklies are actually monthlies and naming convention is FUBAR.
Quote from ammo:
they are not pivots per se but it was a low from the previous day and the spot where price visited most often 2 days before