Thanksgiving Day is no longer all about turkey: It's eating away at Black Friday shopping.

U.S. shoppers spent $9.74 billion at stores on Black Friday, the day after Thanksgiving. That's a drop of 13.2 percent compared with the same day last year, according to preliminary data released on Saturday by research firm ShopperTrak.

The decline appears to be because more Americans were shopping on the holiday itself: Combined spending on Thanksgiving and Black Friday, which had previously been considered the official start to the holiday buying season until this year, rose 2.3 percent to $12.3 billion.

The data shows that Thanksgiving, which along with Christmas had been one of only two days that most stores are closed each year, is becoming an increasingly important shopping day for major retailers.

Black Friday used to be the time when retailers would open early and offer deep discounts to draw shoppers into stores. But a few retailers started opening on the holiday in the past couple of years. And this year, at least a dozen did so, with a few opening earlier in the holiday than they did last year.

The National Retail Federation predicted that 33 million, or almost a quarter, of the 140 million people who planned to shop during the four-day holiday weekend that ends today, would do so on Thanksgiving. And analysts had questioned whether the holiday openings would steal sales away from Black Friday or result in people spending more overall.

"Retailers were pretty successful in drawing the consumers into the stores on Thursday," said ShopperTrak founder Bill Martin, who's company counts how many shoppers go into about 40,000 stores in the U.S. But "Thursday's sales came at the expense of Black Friday's numbers."

The decline in sales on Black Friday was the second one in a row. Last year, sales on Black Friday dropped 1.8 percent to $11.2 billion, though it still was the biggest shopping day last year, according to ShopperTrak.

Despite the big drop in sales this year on Black Friday, ShopperTrak's Martin said he thinks it will remain the biggest shopping day of the year for the 10th year in a row.

But if retailers continue to promote Thanksgiving as the start of the holiday buying season, he thinks the holiday will eventually surpass Black Friday in sales.

"We're just taking Black Friday sales and spreading them across a larger number of days," Martin said.

There will be a clearer picture of sales for the first holiday shopping weekend today when The National Retail Federation releases data.

Overall, the retail trade group expects sales to be up 3.9 percent to $602 billion for the season, which encompasses 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.

So far, some retailers said the holiday shopping season was off to a good start. Wal-Mart said it sold 2 million TVs and 1.4 million tablets on the holiday, while Macy's said 15,000 people showed up to the 8 p.m. opening of its flagship store in New York City on the holiday.

Edwin Molina, 30, a New Yorker who works in construction, waited with his wife in line for an hour on Thanksgiving at a Best Buy that opened at 6 p.m. on Thanksgiving. Among the things he bought: a Samsung tablet computer for $250. He said he liked the earlier hours compared with last year's midnight opening.

"It was better, less hectic," he said.

But not everyone liked the idea of the earlier start to the shopping season. Workers' rights groups and some shoppers led small protests to decry the way some store employees were forced to miss holiday meals at home.

And as his wife shopped in a mall in Atlanta's northern suburbs on Friday morning, Curtis Akins, 51, sat on a bench, lamenting the holiday openings.

"It's taking away from the traditional Thanksgiving," Akins said.

AP reporters Jeff Martin and Mae Anderson contributed to this report.