Don't ask me how they figure it out, but here is some sales report data for Thanksgiving & Black Friday 2019.
Note: these are obviously only "estimates". This is regarding all product, NOT only fashion.
Adobe Systems found that shoppers spent 4.2 billion dollars online on Thanksgiving Day. That was a 14.5% increase year over year and a new record high with 65% of digital orders placed via a mobile device (smartphone). The digital spending continued on Black Friday to reach $7.4 billion as per Adobe. By the way, if you want to do the math that means Black Friday was $3.2 billion.
Data from ShopperTrack found that brick & mortar sales rose 2.3% on Thanksgiving, but fell 6.2% on Black Friday.
Data Source Nasdaq Apparel Search Retrieved 12/2/19 from article at: https://www.nasdaq.com/articles/daily-markets%3A-investors-seeking-tangible-trade-progress-black-friday-data-comes-in-2019
Adobe Analytics: discover consumer spending trends to understand how
they are buying this season.
You may have interest in learning the difference between Black Friday & pre-Black Friday.