3:14:59 PM CST to be more precise. This is the last second that you can enter your order to be executable. At 3:15:00, orders are no longer accepted. You have to wait until 7:30 PM CST I believe to trade them now that there is extended trading on the SPX options available.
Expiring SPX PM settled options will stop trading at 3pm ct. The non expiring ones will continue to trade until 315pm. Note that a "curb" session will also begin at 315pm ct and these options will continue to trade for another 45 mins, although most brokers don't seem to allow access to this curb session.
Yeah I noticed this "curb" session. What is the session for? Why don't we have access to it? I asked this question before but I never got an answer.
The curb session started fairly recently. I think the CBOE was trying to match trading hours with the SP futures. Why you don't have access really depends on your broker. Mine is working on it, hopes to have access in a couple of weeks. Haven't tried IB yet, they offer access to the overnight session, so they may offer access here. Trade SPX and VIX Options Nearly 24 hours a day (cboe.com)