So I heard people in US have black friday

What it the moment when all prices are revised and reduced? Is it 00:01 AM?

It's up to the retailer. It's called Black Friday because, as the busiest shopping day of the year in the US, it's supposed to be the first day the retailers show a profit (ie, they're "in the black" as opposed to "in the red" referring to the color of the ink on the loss and profit sides of the ledger) after factoring in annual expenses (including their cost of the items to be sold) and taxes. Having just celebrated the Thanksgiving holiday, Americans turn their attention to the Christmas season and begin shopping for gifts. Unfortunately, this year a lot of discount retailers (whose merchandise is usually produced in places such as Bangladesh and Southeast Asia under sweatshop conditions and is of lower quality) are trying to get ahead of the game by opening their stores on Thanksgiving Day rather than closing for the one day and allowing their employees to spend the day with their families.

I'm not promoting Staples, but they have excellent Black Friday deals.

What I like best is the way they manage it here in my town. No one has to stand outside. They open the doors a couple hours before the sale starts and let everyone in. As they come in they ask if there was something you wanted to buy that was advertised...if so, then they give you a ticket for that item. When the tickets are gone, then the allocated stock for that sale item is gone. This is for the big expensive items or limited number items that don't have rainchecks for later purchase.

The bag is a big old fashioned grocery bag, and you can stuff it as full of stuff as you can fit in it...and it is 20% off. If you are a rewards member, you might get even more off.

I've picked up things on the BF sale such as a 23" LCD monitor for only $89, 16gb flash drives for $4, etc... The best deals are normally those in-store items that are not advertised in a sale paper. Some things are obviously overpriced...but others are great...you never know until you show up.