The SAG contract expires tonight at midnight. They met every day this weekend and worked into the night to come to an agreement but thus far they're not quite there yet. Even though the contract expires tonight, though, doesn't mean that they're automatically on strike. They'll continue to negotiate for the next 8 days. If the AMPTP doesn't give SAG a desirable deal and SAG won't budge, then SAG will need to ask its members to vote whether they wanna go on strike or not. They would need 75% of the members to vote yes.
I can tell you that most people aren't expecting a strike. In fact, the word strike has barely been mentioned, except by the media, which is trying to make this way more dramatic than it actually is. Of course that doesn't mean a strike won't happen, but it would take much more money/effort/time to organize a strike at this point and neither SAG nor the AMPTP want that to happen. The producers and studios already lost way too much money earlier this year and neither the industry nor the state of California can afford another strike. It would completely fuck everyone up. But unlike the WGA negotiations, the SAG negotiations seem to be going very peacefully and a lot of people are very positive about it. It's just a lot of wait and see. Cross your fingers.
There are also some issues with AFTRA, but that makes things way more complicated, blah blah, just Google it.