When the first settlers came to America, they barely made it through the Winter. Many died, and those who survived, survived solely by the grace of the Native Americans hospitality.
In the spring, the Native Americans taught the settlers how to cultivate crops.
When the settlers brought in their first harvest, they held a large celebration, which included their Native American neighbors, to celebrate the fact that they could provide for themselves, giving thanks for the help provided and the generous harvest that would get them through the next winter.
We celebrate Thanksgiving in memory of that feast, to remember to give thanks, and that we are blessed.
It's also become the gateway to the Christmas season. Since it falls on Thursday, most people take a 4 day weekend and begin their Christmas shopping on Friday. Stores put up Christmas decorations, and things are festive for the next 30 days or so.