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I can't figure out the logic behind SPX option data for 2008-2009 years.

First, all traditional SPX options have exp_date on the third Saturday of each month. How can it be? Why not Friday?

Second, I have traditional and end-of-month option series, but not Weeklys. Is my dataset incomplete, or were they not introduced back then?

Third, the list of exp_dates doesn't make any sense to me. For example, on 17-Jun-2008 I have traditional options with expirations in: Jun 08, Jul 08, Aug 08, Sep 08, Dec 08, Mar 09, Jun 09, Dec 09 and Dec 10. And EOM options expiring in Jun 08, Sep 08, Dec 08, Mar 08. How does CBOE decide, which options to list?

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    $\begingroup$ Friday is the last trading day before expiration, while Saturday is the time that the expiration actually occurs (when the exchange declares the options to be expired). However this is a small distinction of interest to no one except maybe some lawyers. $\endgroup$ – Alex C Apr 8 '16 at 22:03
  • $\begingroup$ If you look at this chart you will see there was no volume in weekly options before June 2010. cboe.com/publish/WeeklysQuarterlyscharts/… $\endgroup$ – Alex C Apr 8 '16 at 22:10
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1 - For historical reasons options expire on Saturdays, before noon. This has to do with potential reconciliation issues that are largely gone nowadays because of back-office automation. Practically, you can think of SPX index options as expiring at index settle, which is 4:00 pm EST on Fridays for PM expiring options, or 9:30 am EST on Fridays for serial options.

2 - Weeklys were around since 2006, your dataset is incomplete.

3 - Those are actually quarterlies, the only EOM options before 2014. And the four months you listed are indeed the only months for quarterlies. I assume that the reason for this choice is that they correspond to standard futures expirations months.

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