No, this is not a religious observance. Not, at least, unless you worship Mammon, like the people in this story.
Anyway, back in the day, businesses kept their accounts in ledger books with profits written in black ink and debits written in red ink. If a business had more debts than income, it was running in the red. If it was going further and further into debt and losing money like crazy, it was bleeding red ink.
A lot of retail businesses run close or run in the red all year. They expect to.
BUT the day after Thanksgiving is traditionally (we’re talking about the United States of Retail…er…America, here) the first shopping day of the Christmas season, and the biggest shopping day of the year. Retail businesses literally bank on making so much during the Christmas buying season that they make up their deficits and turn a profit. In the old days, their accounts would go from red ink to black ink.
Black Friday. It’s a good thing, not a bad thing.
writing prompt: Send a character out to shop on Black Friday not realizing it’s the busiest shopping day of the year.