I looked at the CBOE website and they say the expiration is the Saturday following the third Friday of each month. However, I look up an options chain for Google, for example, and I see three expiration dates for May: May 3, 2013, May 10, 2013, May 18, 2013.
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Standard Options on CBOE expire on the Saturday following the third Friday of a month. Additionally to that there exist weekly options. That's why you see these two series of options.
