You can thank the Macy’s Thanksgiving Day Parade for that! It started in 1924 as a way to promote the store’s Christmas sales. Over time, people thought of Thanksgiving as the start of the holiday shopping season.
In 1939, business owners convinced President Franklin D. Roosevelt to move Thanksgiving to the third Thursday in November, making the shopping season longer. But that led to confusion, as many people had made plans to celebrate a week later. In 1941, Congress moved it to the fourth Thursday in November.