So you started the eggs on Saturday, June 12. That means the 21 days are up on Saturday, July 3. That 21 day thing is not an absolute, it's just a target. Don't be surprised if they hatch a couple of days early and (the harder thing) don't freak out if they are a couple of days late. Yeah, sure. You are going to panic if they are a bit late, that's human nature, but unfortunately they are sometimes late. So stock up on ice cream and have a bowl when the pressure starts to get to you. Or if they are early or on time, have a party. Hopefully it is more than one chick.
I don't know what your incubator looks like, there are many different makes and models. On Wednesday, June 30 (tomorrow) go into lockdown mode, whatever lockdown looks like with that incubator. Stop turning and increase the humidity. Then be patient. Try very hard to not interfere.
They can live for over 72 hours after they hatch. They absorb the yolk before hatch and can live off of that. That's why they can be put in the mail. You don't have to be in a hurry to take them out. But sometimes after they dry off take them out and put them in the already prepared brooder.
If you have any questions come back and ask. When it is over tell us how it went. Good luck!