Quote from option911:
Standard futures options expire into futures just like an equity option expires into the underlying, same ramifications. Except during the quarterlies. On the quarterlies, the futures options expire like a cash settled index. This is probably your confusion.
So the quarterlies-March, June, Sep, and Dec expire into cash on the morning print of expiration friday-they do not trade on exp friday. Basically The ES an the SPX are the same contract (save multiplier size). The serials jan, feb, apr, may, jul, aug, oct, and nov expire into the futures contract. Thus they trade on expiration friday and are different. So this month, you have until friday night.
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Re: Serials, Do they expire into the next (Dec) futures contract or 500 shares of SPY?