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The week after Christmas, which includes six shopping days, should be strong, but it’s being compared with seven days last year because the week, as retailers measure it, ends on Saturday.

As proof that consumers are timing their spending to when they know they’ll get the best bargains, Black Friday, the day after Thanksgiving, was the biggest sales day, as expected, generating sales of $11.4 billion, up 6.6 percent from a year ago, according to ShopperTrak.

But based on preliminary data, Christmas Eve and Dec. 26 were the second- and third-heaviest spending days of the season, according to ShopperTrak founder Bill Martin. He originally had expected Dec. 17 to be the second-largest. Christmas Eve wasn’t even forecast to be among the top 10 days.

“Shoppers are willing to spend when they know the biggest discounts are available,” Mr. Martin said.

ShopperTrak measures foot traffic in 25,000 stores in the U.S. and blends those figures with economic data along with proprietary sales figures from merchants. The data excludes sales from auto dealers, gas stations, restaurants and grocery stores.

A fuller holiday spending picture will come Jan. 5, when stores including Target Corp. and Macy's will release December sales figures.