I have never liked the idea of people putting up Christmas decorations before we have even finished celebrating Thanksgiving. The holidays have meant a lot to me over the years, and to see someone bypass Thanksgiving for Christmas really rubs me the wrong way.
Thanksgiving is the perfect holiday to unwind. You are not pressured into giving gifts like you are at Christmas, for one. When someone has a Christmas tree up at Thanksgiving dinner, it casts a shadow over the entire holiday. The Christmas tree makes it clear that the person hosting is already prepared for the next holiday, bulldozing right past the tenderness of Thanksgiving. 
There are plenty of Thanksgiving decorations you can use instead of a Christmas tree.
The table is the center of the Thanksgiving celebration, so a centerpiece on the table is a perfect opportunity for holiday décor that a person can have a lot of fun with. Pumpkin decorations make for a lovely display of the Thanksgiving season. You can even put up a cute little turkey and surround it with fall leaves and pumpkins.  
One time, while at my cousin’s house for thanksgiving, I noticed she had her Christmas tree out, and I asked her why.  She told me that she simply could not wait for Christmas.
I know my cousin enjoyed the family company of Thanksgiving, but this made me feel as though she did not care as much for the holiday as she did Christmas. For that reason, I have always made sure I properly celebrated Thanksgiving before I set out any Christmas decorations. 
Thanksgiving is a holiday that can be just for the family. Christmas has the stress of gift giving and can be a very stressful time for some people, while Thanksgiving can just be a time to relax and visit. 
The annual day of thanks can be reserved as a time for people to unwind before the hectic times of finals and Christmas preparation. While the holiday parties and shopping build plenty of memories, but they do not warrant blowing past Thanksgiving — a time to get recharged with the turkey, dressing and stories around the table.
The idea of putting the Christmas decorations up before Thanksgiving diminishes the Thanksgiving spirit, so it is much better to just leave the ornaments and tree in the attic until the turkey has been eaten.