George Washington certainly felt that the celebration of Thanksgiving could not be divorced from any religious connotation, as indicated in the following Thanksgiving Day proclamation:
"It is the duty of all Nations to acknowledge the providence of Almighty God, to obey his will, to be grateful for his benefits, and humbly to implore his protection and favors."
Although certainly not as revered as the Birth of Christ, it is as much a religious holiday as Christmas, Easter or Passover. Unfortunately this aspect of Thanksgiving has been downplayed in our schools and replaced with teachings about tolerance and ethnic diversity. They have been so successful at this there is no wonder that when it comes time to talk about Thanksgiving in the classroom, virtually no one opposes. This is because the celebration has become completely divorced from its true meaning, and hence is uncontroversial to the secularists.
They have not been so successful with Christmas, however the attempts to divorce this holiday from all historical meaning are well underway. We are seeing 'Christmas' replaced with 'Holiday.' It is done in the name of tolerance and diversity awareness.
It was in 1789 that George Washington gave the above Thanksgiving Day Proclamation. By 1989 its celebration has already lost much of its true meaning as taught in the schools and celebrated by many. I wonder what will be the fate of Christmas in the year 2089?