You can. No one will say, "Oh my gosh, that's wonderful turkey you're serving us,"* but it's certainly better than having it go to waste.

You do have to follow general safety requirements. You can't freeze turkey that has been left out on the counter for hours and hours. If the turkey is good when it goes in, it will be safe when it comes out of the freezer. If you can freeze it in an airtight container and you have some means of removing all the extra air from the container (that is, if you have a vacuum-bag system), your turkey will last longer and fare better.

Thaw it in the refrigerator. You should probably consume it within a month. Beyond that, the texture will suffer so much, you will be unlikely to want to consume it. If you're going to reheat the turkey, it's best done in some gravy or sauce, where it does not come in direct contact with heat. You will probably want to use it in a soup or stew in any event, where the detrimental transformation it undergoes in freezing and thawing will not be so noticeable. You will almost certainly be disappointed if you try to use it for cold leftover turkey sandwiches.

stated elsewhere at Ochef, freezing doesn't threaten the safety of foods – cooked or uncooked – but the texture can suffer mightily through the freezing, thawing, and recooking processes. Some people don't mind very much; others find it unappealing.

*Unless they're exceptionally polite, in which case you should invite them over as often as you serve leftovers.