Most traders believe that “expiration Friday” is the true day that option contracts expire. That has never been true. Expiration occurs on the Saturday that follows the 3rd Friday.
However, next year expiration officially moves to Fridays.
You may ask: How is this important to you, the options trader?
Answer: It is of no importance. But you may want to learn the rationale behind the move.
Here is the official definition of “expiration date” according to the CBOE.
Expiration day for equity and index options is the Saturday immediately following the third Friday of the expiration month until February 15, 2015. On and after February 15, 2015, the expiration date will be the third Friday of the expiration month.
To understand why expiration was originally set to Saturdays, read my explanation here.