This link is on Thanksgiving. Apparently the festive season in the United States starts with this festival. It falls on the last Thursady in November. Details about this festival are available in this link.
In the Northern Hemisphere, where winters tend to be bleak and is known to cause depression, festivals of fun and celebrations, tend to bring people together and help them ride out a period of hibernation in a manner of speaking. It is also an occasion when agriculture is impossible and possibly, a lot of outdoor activity. The reasons for spending on gifts, get- togethers, possibly provide sustenance to the economy by driving the demand side and generate some degree of incremental economic activity that provides employment in the service and entertainment sectors.
What starts with Thanksgiving goes on till Christmas which is at the end of December.