This could be the year when Black Friday is dethroned as the biggest shopping day of the year.

At least that's the prediction of Bill Martin, the founder of ShopperTrak, a Chicago firm that measures store traffic and sales. He has become a guru of sorts when it comes to predicting and measuring the ebb and flow of the holiday season.

According to ShopperTrak, Black Friday has reigned as king of the holiday shopping season — and entire year — for every year since 2005 by bringing in the most sales on that day. But its supremacy has begun to wane as more stores have been opening Thanksgiving night, and a tipping point is at hand.

Macy's has already said it will open at 6 p.m. on Thanksgiving this year. Other major retailers have not yet announced their hours for that day, but Martin expects that more will follow suit.

"Our numbers show over the last three years that Thursday sales are growing at a pretty rapid pace," he said. "It's leaching sales from Black Friday."

He expects the last Saturday before Christmas, often referred to as Super Saturday and Dec. 20 this year, to be the biggest shopping day this year.

No overall bump in sales

Black Friday should still slide in at the No. 2 spot, he said, followed by the day after Christmas, which falls on a Friday when many people should be off from work.

ShopperTrak measures in-store traffic and purchases, and does not include online sales in its forecasts.

Martin, who was in Minneapolis for the Shopper Marketing Conference & Expo this week, said Thanksgiving-night openings are not giving retailers an overall bump in sales. They are just taking away sales from Black Friday.

"Retailers say that consumers are clamoring for them to be open on Thanksgiving, but that's not the case," he said. "They're just attempting to get to the wallet before the money is gone. That's what this holiday creep is all about."

ShopperTrak will finalize its holiday sales forecast in the next week or so, but Martin said he is expecting it will project an increase of 2.5 to 3.5 percent for in-store sales. That is a bit lower than the National Retail Federation, which has put out a rosy forecast of 4.1 percent increase for holiday sales. Martin added that he thinks the NRF's number is a little optimistic. "Growth is slowing a little bit, but growth continues to prevail," he said.

Also, he said to expect to see retailers being more aggressive in November with promotions. It's part of a decadelong shift to more holiday sales that month as retailers try to clinch sales earlier, he said.

"November continues to grow as December declines," he said. "That stroke of midnight after Halloween, you're going to start seeing the Christmas promotions."