Okay, it’s too late to vote by mail. 
Lots of people are going to vote in person at early voting locations, but millions more will be voting in person on Election Day.
Question: if people are still waiting in line at the polls’ official closing time, will they be turned away or allowed to vote? IOW polls are supposed to close at 7:00 pm and 1,500 people are still in line. What happens?
I assume each state has its own set of rules. However, I can’t remember any reported instances of people in line at closing being turned away. Officials will go to the end of the line at closing time and station somebody there to turn latecomers away. Everybody else in line will get to vote.
Sometimes, states will extend hours for particular polling places in special circumstances, but that can’t be predicted ahead of time.
There may be exceptions to this but generally speaking if you’re there, you vote.
ETA. And it’s not too late to vote by mail in some states. They allow ballots to be counted if they are postmarked by election day. Check your state’s rules.
According to # 3 on this list (and from what I’ve heard elsewhere) if you’re standing in line when the polls close, then you’re “grandfathered in”.
I wouldn’t want to be the person that has to start turning away people who walk up from that moment forward though.
That depends on the state.
I know in Ohio in 2000 some people waited in line till midnight. I think it is pretty standard to allow anyone who arrived before the polls closed to vote. But it will, of course, be determined by state law (or practice if the law is silent).
But ballots that come in before election day count more because they are less likely to throw the election into planned chaos when certain parties try to stop the vote count.
Every state I know of provides that those already in the line at closing time get to finish.
Important to check each rule – postmark on or before election day is the usual norm, but some may also have a cutoff received-by date afterward.
As well as the locale. Even if Washington required receipt by election day, I have to believe that the ballot could make it six miles downtown in the six days between now and Tuesday. I know there’s been monkeying with the post, but my general experience is mailing something in-town gets there next day. Of course, not everyone’s ballot goes somewhere in town, but I’d imagine they generally do if you live in the county seat.
Or even sooner, like requiring ballots to be received by Election Day. In my state of Connecticut, for example, ballots must be received by 8:00 pm on Election Day. However, if you place your absentee ballot in an official drop box, that counts as being received.
They actually added an insert to the absentee ballot instructions this year explicitly telling voters to place their absentee ballots in the official drop boxes before 8:00 pm on Election Day to ensure that ballots are received on time [instead of mailing them back via the USPS].
Fortunately, we have a lot of drop boxes in Connecticut. Each town has one, and there are 169 towns in Connecticut. (Connecticut has no unincorporated territory in the state, nor is there any county government.)
In Maricopa county the early ballots are counted after the signature is verified, but the results won’t be released until the polls close on election day. I don’t know if the other counties are doing the same but Maricopa county is 61% of the state’s population and the registrar is predicting 90% of votes will have been early when all is said or done – the primary was 70%.
There was a news story on Saturday saying the county registrar was predicting three locations in the county will be facing long lines on election day. All three are Republican enclaves. My schadenfreude kicked in so hard it hurt.
I dropped my ballot off in a drop-box at city hall on Friday. Yesterday I checked and found it received and accepted. 
Yeah, every state I’ve voted in lets you vote so long as you join the line before “the polls close”. I once voted shortly after the polls close in NYC, because i got there late (but before polls close) and even though the line was short, it moved glacially slowly. I was the last voter. They closed the doors shortly after I arrived. But it was always clear that I would be allowed to vote.
Every state is going to have a received-by date cutoff as well, because at some point you have to finalize the count. Given how long there is between Nov 3 and Jan 20, there’s plenty of room for that cutoff to be way longer than the expected travel time of a ballot, but it may or may not actually be that long given the state of the USPS right now.
I’ve been the last voter at my polling place, too. I got there once at 7:55 pm. Nobody was there except the poll workers, and they had broken down all the privacy booths but one. I didn’t want to hold them up, so I voted as fast as possible, turning my ballot in to be scanned a minute or two after 8:00 pm. To their credit, nobody rushed me or anything.
You are entitled to vote if you’re in line when the polls close. However, I volunteered as a poll monitor in 2016, and the poll workers at the polling station I was at didn’t know that. They tried to shut down at 8pm sharp with almost 200 people still in line. I had to get in their faces and demand to speak to their supervisor (is it still Karen-ing if it’s in service of democracy?) and finally they gave in and stuck around. It’s really important that people know their rights; a lot of folks in line seemed to think they were out of luck when the poll workers started shouting that they were closed.
I fully expect fake officials to show up at various polling places and tell people in long lines that the polls will be/are closed so they might as well go home.
Use one of our (WA’s) many conveniently located drop boxes, and you can cast your ballot even on election day and know it arrives in time. 
Yeah, I already did. And confirmed it’s been received and counted. My point is that for a lot of people - such as myself - there is still a realistic expectation that anything mailed today would arrive before election day.
If you have a mail-in ballot, or can get one, you should be able to drop it off at your local clerk’s office. At least that’s how it works in my state-- you can drop off your ballot up to 8pm on election day. That’s what I’m going to do in order to bypass the mail system.
NC has always allowed anyone in line at closing to vote. 7:30 is the closing time here. A few times it was extended due to problems at some poling places.
Funnily enough, I remember this happening somewhere just about every election — due to poorly-trained poll workers or skullduggery, who knows?
It seems like an election-night-reportage cliche: observers argue with the poll workers and end up getting a judge involved, who sides with the people in line, yadda yadda.