An oft believed myth which I did believe myself. However the data does not lie! Per this article on Quartz, it is not actually even all that close. I’ve heard elsewhere that summer typically has 5-10 days busier days than the day before Thanksgiving.
A summation of their findings from the 2013 data: the day before Thanksgiving ranked 27th. The Friday before Christmas was the busiest winter day, coming in at number 4. The top 3 spots were all made up of Thursdays in summer in July. On one hand the variance may not seem all that large – 39,753 flights for #1 and 38,676 for the day before Thanksgiving – less than 3% more for the top spot. Then again that’s still 1,077 more birds in the air.