Popular explanations reference the color of ink used to record store profits, slave auctions and a general feeling of doom among employees.

Walmart will be closed on Thanksgiving but most stores will open at 6 a.m. local time on Black Friday. Target will be closed on Thanksgiving, but most stores will open at 6 a.m. local time on Black Friday. Macy’s will be closed on Thanksgiving, but most stores will have extended hours from 6 a.m. to 11 p.m. on Black Friday.

Before holiday shopping mania fully sets in, most mail carriers are taking a break this week. The United States Postal Service will not be operating Thursday as the agency observes Thanksgiving, a federal holiday.