I have relatives who frequently come to Thanksgiving + Xmas dinners each year.

The questions is: Is it good manners or etiquette to bring (as an invited guest) your own tupperware containers to a dinner with the "assumption" that you will be filling them up and taking them back to your home to have "leftovers" later in the week courtesy of the host(s)?

I know the way I was raised this is extremely tacky behavior.

I would find this offensive if I was the host. I, as the host, should have the decision whether to offer leftovers, and then to provide foil to wrap them up.

Although I would assume it is proper etiquette for a host to offer the leftovers, it isn't proper etiquette for the guests to assume that they will.

I'll agree with the answer above. The host should benefit from the leftovers since they went through the effort of buying the supplies and cooking the meal. The guest should bring the wine :) .

Oh my now I find that extremely rude I'm afraid id be inclined to telling them where to stick there tubs. I don't know how some people can have so much gall.

I agree, it is pretty thoughtless, however in my social circle a holiday is not the time to fuss over it... later, it can be a private discussion. Or for the far more courageous than I LOL, the host can simply thank the guest for the gift of more tupperware (god knows, we could always use more!) and then simple put it away for the duration of the celebration. That may take more than once to make the point though LOL since tacky is usually slow too.

I will agree this is not proper, unless this is some pot luck affair where everyone is bringing dishes,

You have basically three choices, don't invite them,

Invite them but tell not to bring dishes,

Forget it, since it appears this has been going on for some time now.

Change over and have diner at there house part of the holidays

I'm pretty easygoing, but I'd definitely have to agree with the above posts. :)

I would call them up after the holiday was over and tell them that it was rude and if they ever tried that again, they would be invited back to my house.

The HOST buys the food, cooks it, THEY keep the leftovers.

Now, if they want to offer leftovers to the guests, that is their choice, but the guests should NOT assume they are getting any.

Yes, it is extremely tacky. Unless the host told you in advance to do that.