@Boerenkool, @Sam and @ClaudnDaye did a truly terrible job at educating you on the major differences between the dinners at Thanksgiving (which some Native Americans call Thankstaking) and Christmas, (which, as you probably know, should likely be celebrated in September). So I see I have to greatly supplement their information.
As @Sam explained, the Xmas dinner is really an extra: the main focus of the day is the gifts. The meal, well… it just has to be large, rich, and difficult to digest.
The Thanksgiving meal, on the other hand, is the primary attraction of the day, and, as @ClaudnDaye hinted, followed a very precise ritual, including specific dishes that MUST be there. In the same way as the Jews celebrate a precise ritual for Passover, which varies depending exactly upon where your family came from, but which is invariable every year, the Thanksgiving dinner must follow its own extraordinary specific rules, that vary depending upon your family, but that must followed TO A T. I will give a brief summary of the mainstays that are generally accepted (although there are exceptions):
-
The primary meat must be a turkey, in general presented whole
-
There must be large quantities of rich mashed potatoes. Different sects of Thankstakers have different beliefs as to exactly how they must be made. Some, for instance, believe that they must be chunky. Others believe they must be smooth. Each Thankstaking sect excommunicates the others. Almost all, however, believe that the ratio of potato to butter should be about 1:1 on that holy day
-
There must be gravy, turkey gravy, in large quantities. Most Thankstaking sects excommunicate those heretics who buy the gravy already made. In some families, a part of the ritual involves some of the cooks crying after burning the gravy, or spoiling it in some other way. Not having gravy at Thankstaking is a criminal offense in America (and certainly for my wife).
-
The dinner must finish with pumpkin pie, a sweet pie with a very thick layer of cooked pumpkin, with large quantities of cinnamon (do remember that American cinnamon is NOT the cinnamon that you know in Europe, which comes from Ceylan, but a different plant, much stronger but less subtle, coming typically from China, made from Cassia plants).
-
Most families believe that you must have table decorations of Native American corn and small decorative gourds and pumpkins.
-
And, last but not least, the eating on Thanksgiving must be followed by a week long of turkey sandwiches and leftovers, which are as almost important as the original day
You can see, @Boerenkool, that the ritual is critical! Many a family has fallen apart over disagreements on Thankstaking theology. As for me, it may well be my very favorite celebration 
The food is great, everybody is happy, and, somehow, every year adds to the previous years’ wonderful memories to aggregate a fabulous aura of what Thanksgiving means!