When I sit down at the table, do I have to wait until the host puts their napkin on their lap before I do?

​The answer to when do you put your napkin on your lap depends upon the situation.  If you are at someone’s home, you wait until the host or hostess puts their napkin on their lap or invites you to do the same.  This is a sign of respect.  The same goes for when you are invited to a meal in a restaurant by a host.  However, if  you are at a table at a meal without a designated host, you should put your napkin in your lap as soon as you sit down.

If you know that Grace will be said before the meal, the napkin waits on the table until Grace is finished and then it goes on your lap.​

Where do I put my napkin when I have to leave the table during a meal and will return?

There seems to be some controversy about whether you leave your napkin in your chair, lightly folded or put it on the left of your plate on the table.  Plus, there are those who think it doesn’t matter where you put it.  But it does.  There is a silent service code that servers follow to make sure your meal goes the way it should.  The serving staff knows that if the napkins are off the table and in the laps of the diners that the meal has begun. Likewise, if the napkin is back on the table (to the left of the plate in typical dining circumstance and to the right for formal dining) then the diner has finished their meal and is not returning.  If the napkin is left on the chair, either on the arm or seat, the indication is that the diner will return and not to remove their plate.  Even if there is food left on the plate, the server will still assume the meal is over for that diner if their napkin is back on the table.

But you say, the napkin will dirty the chair and why would you want to do that?  This is where you, the diner, have to actually look at your napkin before you leave it, and fold it lightly making sure the soiled part doesn’t touch the chair seat.  If someone tells you that it is gross to use a napkin that has been where your bottom has been, the best response is to tell them that your clothes are on and your bottom is not actually exposed to the material of the napkin.