Bluerose wrote:Black friday?
Theories of origin
 Accounting practice
One theory is that retailers traditionally operated at a financial loss for most of the year (January through November) and made their profit during the holiday season. When this would be recorded in the financial records, common accounting practices use red ink to show negative amounts and black ink would show positive amounts. Black Friday is the beginning of the period where they would no longer have losses (the red) and instead take in the year's profits (the black).
Earliest Citation, speaking to the Friday after Thanksgiving:
A BLACK FRIDAY.
There have been many Black Fridays in recent history. Most of them have been days of financial panic. There has been none of blacker foreboding than last Friday. And the blackness is not loss or fear of loss in stocks and bonds.
New York Times (1857-Current file).
New York, N.Y.: Dec 3, 1922. pg. 38, 1 pgs
Look up in the red, in the black in Wiktionary, the free dictionary.
 Stress from large crowds
Another theory comes from the fact that shopping experience on this day can be extremely stressful. The term is used as a comparison to the extremely stessful and chaotic experience of Black Tuesday or other black days. According to The Word Spy:
Christmas decorations around Tampa Bay started going up in late October, and business has been brisk since then. And while Friday--known as Black Friday for the legendary hordes--will be the biggest shopping day for many area stores, others ring up the greatest sales the Saturday before Christmas.
—Marilyn Marks, "Retailers expect good sales this Christmas," St. Petersburg Times, November 27, 1986
Employees of retail stores have for years referred to Black Friday in a satirical way, to note the extremely stressful and hectic nature of the day. Heavy traffic and customer demands added to the long hours make it a difficult day.