Millions of Americans are expected to head to the stores for holiday gift shopping this Thanksgiving in what's quickly becoming a new holiday tradition.

Just a few years ago when a handful of stores began opening late on the holiday, the move was met with resistance from workers and shoppers who believed the day should be sacred.

Last year, more than dozen major retailers opened at some point on Thanksgiving evening. And this year, at least half of them — including Target, Macy's, Staples and J.C. Penney — are opening earlier in the evening on the holiday.

Bill Martin of ShopperTrak is expecting a sales increase of 3 percent to 5 percent on Thanksgiving. And the National Retail Federation expects more than 25 million shoppers to take advantage of the Thanksgiving openings.

At the same time, analysts say that's likely to take a bite out of Black Friday business.