+1. i've said the below story on other options threads on et re this exact same issue (selling otm calls for "free" premium" esp so close to expiry). a company (not sure the name anymore) announced it was being acquired at 4:01 EST on expiry day. the problem for those who sold a boatload of otm calls for like 0.05 was that those calls were exercised after the market closed and were worth something like $20. can you imagine selling 1k of those calls friday at 3:30 EST, going home and thinking "another easy day selling premium" then coming in monday finding out you owe 1,000*20*100.
The remaining premium reflects a finite probability that something "weird" can happen from Friday's close to the Saturday,(at noon?), expiration.
Technically while the option expires on Saturday the OCC requires instructions by Friday at 5:30 or 6:30. I can't remember the time. There was one trade I did where the govt was going to issue a report on Medicare on Friday evening. Expiring options priced in nothing. It was essentially free gamma. But there's a reason why premium should exist. Just because an option is out of the money, doesn't mean you wont get exercised. You aren't trading max(s-k,0) but rather the right to buy stock which doesn't have to be optimally done. This risk is worth money for an option that's a little out of the money.
Be very careful scalping time premiums of high priced stocks with interactive brokers if they are close to in-the-money. They will look at what will happen if you got exercised those options and automatically close the options if you do not have enough margin to hold the long/short position. It does not matter that you have enough margin to hold the call/put spread. I have gotten burned a couple of times now with this. Maybe my own fault but it would be nice to get a notice of 1 or 2 minutes to close the options yourself. All I can tell is it seems to be about 15-20 minutes before the close.
The cutoff time is 5:30 eastern time on Friday, but many brokerage houses have an earlier internal cutoff for their customers.