How do you let them know?

Honesty is always best, just be polite!

My mom always uses the fish thing (you know, more than 2 days and they start smelling) but I usually just say that I didn't know they had planned to stay over a certain length of time and have other plans made previously and they will have to leave now.

Just tell them that you guys enjoyed their stay but they might want to be getting back home so your guys can visit them sometime soon and see their neck of the woods. If that doesn't work just politely ask them to please leave so you can get your house ready for Thanksgiving. Or for Veteran's Day. Tell them that you love them but they need to vacate the premises. Judging on their personalities talk to them either humorously, strictly, calmly, or gently. Just be nice, but firm and tell them to get out.

Clear the table. Stop feeding them, and they're likely to leave.

tell them company are like fish after three days they start to stink

You start going around them, telling (as if) to yourself "oh, I got to do....(whatever)". Something like that.

ask them for money , that ought to get rid of them