(ABC News)

Retailers are preparing for a stampede of shoppers on Black Friday, some of whom have already begun camping outside of stores, eager to get the best deals first.

Nearly one in four Americans, 86 million total, hit the stores on Black Friday last year, according to the National Retail Federation.

At one retailer, ABC News witnessed a mock Black Friday scenario, where a steady line of people banged their shopping carts through the door.

There's good reason for the training. Some shoppers in Wisconsin are already camped out, ready to be the first people through the door when the Black Friday sales begin.

"Our time is almost getting paid back just by the savings," one shopper said.

But not everyone is cheerful about the big day. Employees of some big box stores are unhappy their stores are opening on a Thursday for shopping, causing them to miss spending Thanksgiving with their families.

"We're just demanding respect," said Rosetta Brown, a Walmart employee.

Read: Retail Workers, Families Disgruntled Over Interrupted Thanksgiving

Target announced on Monday that it is opening some stores on Thanksgiving at 9 p.m. for its Black Friday sale, which in the past began the day after Thanksgiving.

Walmart will be opening its doors at 8 p.m. Sears will also open at 8 p.m. on Thanksgiving, moved up from 4 a.m. on Black Friday last year. Kmart will be open Thanksgiving Day 6 a.m. to 4 p.m., then it will close and reopen 8 p.m. to 3 a.m. Macy's, Kohl's and Best Buy open at midnight.

ABC News' Susanna Kim and David Kerley contributed to this report.