Option Expiration date oddity

Why are the December 2015 SPY & OEX options using the Saturday expiration date instead of Friday? (There may be others)
Note: that SPY OPRA code is ".SPY151219....", I expected ".SPY121218...".

Thanks if you can point me to info supporting an explanation.

Regards,

 

These options were created before option expiration was moved the Friday.

 

rmorse:
Thank you!

 

https://www.sec.gov/rules/sro/occ/2013/34-69772.pdf