DALLAS – Black Friday didn’t used to be called Black Friday. It was just the day after Thanksgiving.

It was the day retailers had to have holiday decorations up and malls had to get Santa into his temporary housing in center court.

Now it’s the most cutthroat day in retail. The biggest crowds of shoppers show up. And every retailer tries to steal market share from the next guy.

Miracles happen on Black Friday. Stores run out of merchandise. Americans care about saving money. And they develop incredible patience for waiting in line.

This year, some are even predicting the first $20 billion day in retailing.

The world’s largest retailer, Wal-Mart Stores Inc., had its first $1 billion day in the U.S. on Black Friday in 2000. It’s one of the top days, if not the top sales day, for toys, electronics, TVs and apparel. For the seventh year in a row, Black Friday is expected to be the biggest shopping day, a distinction it has swiped from the Saturday before Christmas.

Macy’s, Target, Kohl’s and Best Buy will open at midnight Thanksgiving and stay open for 24 hours. Toys R Us is opening at 9 p.m. tonight. Wal-Mart will have two doorbuster sales, at 10 p.m. and midnight.

Online-only deals go up at 7 a.m. today at ToysRUs.com for the first time. Amazon.com is monitoring others’ doorbusters and adding them to its online store. A plethora of mobile apps make shopping while you’re waiting in line or at home easier than last year’s clunky options.

All these extra store hours and a forecast for Black Friday online sales to be up 35 percent from last year are fueling predictions that Friday may be retail’s first $20 billion day. That compares with $19.3 billion last year and excludes autos.

Americans may have become desensitized by the word “billion,” but $20 billion is still a lot of money, especially when it’s spent by millions of shoppers on cue in a 24-hour period.

To put it in context, it represents the total revenue for Neiman Marcus Inc. in the past five years.

Black Friday has taken another leap with promotions showing up in ads before Thanksgiving.

Competitors J.C. Penney and Kohl’s were sparring on Wednesday. Penney said: Why wait until Thursday? It has “1,000s of specials now” online. Kohl’s for the “first time ever” was giving shoppers $15 for every $50 spent on Wednesday.

Young people think of Black Friday as a competitive endurance sport. Older adults wonder how it evolved.

About 34 percent of consumers 16 and over, or 81 million Americans, plan to shop Friday. That’s up from 31 percent last year, according to the International Council of Shopping Centers.

Joel Bines, managing director of the retail practice at Alix Partners, said he thinks it all started with extended shopping hours in the days leading up to Christmas.

“Then, in 2004 or 2005, big box retailers realized they have people in their stores working 24 hours a day anyway. With incremental labor costs, stores could open earlier and earlier, and why not do it Thanksgiving weekend, too?”

By 2007 and 2008, malls were opening at midnight, and department stores had joined in the 4 a.m. and 5 a.m. openings.

Retailers don’t want to be closed when competitors are open, Bines said.

Earlier this month, Best Buy CEO Brian Dunn said he felt terrible asking his employees to move up the Black Friday opening to midnight.

“If you got all the retail CEOs in the same room, they too would say they prefer not to do it,” Bines said. “But shopping is a 24/7 experience online, and for retailers, these stores are like factories. They want to operate them as much as possible.”

Some Target employees are pushing back about having to come in at midnight and circulated petitions to persuade the chain to rethink its plans.

Most employees understand that “it’s the right thing to do to be competitive,” said Target Senior Vice President Sid Keswani. Target is paying both shift differential pay and overnight pay, he said. “Stores will have food in the break rooms throughout the weekend and other games and recognition to make sure they know they are valued.”

NPD Group Vice President and consumer electronics analyst Stephen Baker said toys were the original catalyst behind making Black Friday bigger the last 10 years. Then electronics took over.

“You can trace the changes in consumer electronics through Black Friday specials,” Baker said. “It was desktop computers, then notebooks, then small electronics like GPS and MP3 players and TVs again.”

When these products can be offered at prices low enough for large numbers of consumers to afford them, they become Black Friday doorbusters, he said. TVs are being offered at or below cost to get shoppers in the door again this year, he said.

“Look for $199 42-inch TVs and $499 prices for 50-inch televisions.”

Keswani, who is about to work his 17th Black Friday with Target, said he recalls the day’s frenzy starting with toys. “It’s become much more intense the last five to six years. Before that it was a few hot toys,” he said. “The one I remember that first brought out the crowds about 10 years ago was the Furby.”

Midnight openings will either become the standard for future Black Fridays or retailers will realize the business wasn’t worth it, Bines said. “I suspect the former.”

If Grapevine Mills, Allen Premium Outlets and Town East Mall, all Dallas-area pioneers for opening at midnight and 1 a.m., are any indication, Bines is right.

Debbie Screws, a spokeswoman for Town East Mall, said the stores want to open at midnight now. Last year, the mall had more than 12,000 people waiting to get in. Town East hands out bags filled with store coupons.

In the past, “we had to beg stores to participate,” she said. “This year it was easy.”