There isn't one. It's an American creation. Black Friday, if I remember rightly, is the Friday after Thanksgiving in the US. Well, we don't celebrate Thanksgiving in the UK.
However, having said that, in the last two or three years companies looking to make money have brought it here. It's called Black Friday, the same as in the States and it's on the same day. It's nothing but a marketing ploy as we have no reason to have it here, but I guess with Christmas soon approaching people see it as an opportunity to get some cheap deals.
It was started in Asda, a supermarket which is owned by the massive US corporation Walmart, and then naturally other shops followed suit because they saw there was an opportunity there. Last year there were some problems with people fighting over goods in the shops and there were several viral video posted online. This year I believe the shops and buyers learned their lesson, as discounts were still offered but it seems most people shopped on the internet rather than on the high street.