In the USA, large retailers like stores at the mall operate in the red most of the year. They make little or no profit for 11 months, if you look at it in a certain bookkeeping way. The last months gross sales in a way represent the stores profit for the whole year.
So the Friday after the Thursday thanksgiving holiday at the end of November is a day off for many Americans. That Friday, known as Black Friday kicks off the real holiday gift shopping splurge in the USA. On that day, the stores accounting is now in the Black, or finally showing profit for the year.
Stores will open stupidly early, and usually have a big pallet of something juicy priced crazy low for the first few shoppers as a loss leader. That's the black Friday sale.
This big sale tradition has now spread to web retailers, so now the whole world might have a good price on things for just one day. So wherever you are, you might find a good price on Nov 29th.