However, it is actually referring to a Friday, where all the prices in the shops DROP.
The tradition started in the US, and now has come to the UK. Shops and online stores will have sales starting from the week before that Friday until the Monday after (called Cyber Monday).
This year, Black Friday falls on 27th November 2015. I'll be looking for a jacket for this autumn-winter transition period.
I hope some sort of Black Friday will arrive in Malaysia too because it's really when businesses gain more from volume rather than big profit margins, and when consumers get the chance to buy what they've been longing for!