Millions of Americans turned out as the holiday buying season started earlier this year on Thanksgiving Day. But there’s still a lot of shopping left.

While figures aren’t yet available on how many people shopped on the first two days of the holiday shopping season, crowds came early and often as more than a dozen major U.S. retailers stayed open for 24 hours or more on Thanksgiving through the day after known as Black Friday.

But overall, The National Retail Federation predicts that 140 million shoppers planned to shop during the four-day holiday weekend that begins on Thanksgiving Day. And of those, about 23 percent, or 33 million shoppers, planned to do so on the actual holiday.

So the challenge for retailers is to keep the momentum going through the weekend — and beyond. In years past, stores have had a robust start to the season by offering deep discounts only to see crowds disappear until the final days before Christmas when the big bargains pop up again.

Overall, the retail trade group expects retail sales to be up 3.9 percent to $602 billion during the last two months of the year. That’s higher than last year’s 3.5 percent growth, but below the 6 percent pace seen before the recession. And retail experts said it’s going to be difficult for stores to get shoppers to keep coming back into stores without bargains.

“Can stores continue the momentum after a promotional November?” said Laura Gurski, a partner and global leader of the consumer product & retail practice at A.T. Kearney, a global management consultancy. “How do you top it in December?”

Despite that there is a lot of the holiday shopping season left, this year may cement the transformation of the start of the holiday shopping season into a two-day affair.

For nearly a decade, Black Friday had been the official start of the shopping season between Thanksgiving and Christmas. It was originally named Black Friday because it was when retailers turned a profit, or moved out of the red and into the black. Retailers opened early and offered deep discounts.

But in the past few years, store chains have been opening on Thanksgiving.

This year, several welcomed shoppers for the first time on Thanksgiving night, while Gap, which owns Banana Republic and Old Navy, opened half its stores earlier on the holiday.

Wal-Mart stores, most of which stay open 24 hours, has for the past several years offered doorbusters that had been reserved for Black Friday. And Kmart planned to stay open 41 hours starting at 6 a.m. on Thanksgiving.

That has led some to question how much further Black Friday will creep into Thanksgiving, which along with Christmas is one of only two days a year that most stores are closed.

“Black Friday is now Gray Friday,” said Craig Johnson, president of Customer Growth Partners, a retail consultancy.

The earlier openings have met with some resistance.

Workers’ rights groups and some shoppers had planned protests on Thanksgiving and Black Friday to decry the way some store employees were forced to miss holiday meals at home. But as of Thursday afternoon, there were no reports of widespread protests.