Jackie Goebel has worked for Walmart for 24 years, but this year, for the first time, she will spend the Thanksgiving holiday working at the retail giant. Like many of her colleagues, she is not happy. "Walmart has become a company so obsessed by the bottom line and greed that it no longer values the importance of the people and families that work for it," she said.

For decades, Thanksgiving has been sacrosanct for Americans – a non-denominational national celebration, and a guaranteed family holiday for most big companies. For retailers, it's also the day before Black Friday, the shopping bacchanal that marks the start of the holiday season.

Now Black Friday is slipping into "Grey Thursday" and taking retail workers with it. Walmart will open its doors at 8pm on Thanksgiving Day. Others, including Sears, Toys R Us and Kmart, will also open at 8pm on Thanksgiving. Target opens at 9pm. Gap is planning to open 1,100 of its stores. Retail experts predict that if this year proves a hit, stores will open earlier next year.

Goebel, 61, lost her daughter to cancer in 2007. She, too, worked at Walmart. This year Goebel was planning to spend the holiday with her husband and her granddaughter. "An experience like that makes you realise what's important," said Goebel.

Sam Walton, Walmart's hard-driven founder, once visited the store in Kenosha, Wisconsin, where Goebel works. She believes he would have been horrified by what has become of his company. "He was a hell of a businessman, but he had respect for the people who worked for him," she said. "To me, this is pure greed."

Walmart workers are planning strike action over the Thanksgiving holiday, supported by the United Food and Commercial Workers (UFCW) and OUR Walmart, an associate led group that in October helped organise the first strike at the retailer in its 70-year history.

A Walmart spokesman said "a very small number of associates" were raising concerns. "While the opinions expressed by this group don't represent the views of the vast majority of the more than 1.3 million Walmart associates in the US, when our associates bring forward concerns, we listen. Associates have direct lines of communication with their management team and we work to understand their concerns," he said.

"Last year, our highest customer traffic during the Black Friday weekend was during the 10pm hour on Thursday. According to the National Retail Federation, Thanksgiving night shopping has surged over the past three years. Most of our stores are open 24 hours and, historically, much of our Black Friday preparations have been done on Thanksgiving, which is not unusual in the retail industry."

Walmart is not the only retailer facing a backlash from staff. Casey St Clair, 24, was so incensed by Target's decision to cancel Thanksgiving for her and her colleagues that she set up a petition on Change.org, lobbying the retailer's CEO for action. It now has over 200,000 signatures.

"Every year the opening gets earlier and earlier. Before long, we will be open all day on Thanksgiving," she said. "Thanksgiving is a day about families, and giving thanks for what we have got. There are a lot of single parents at Target. This just ruins the holiday for them."

She said she had been surprised by management's attitude to the holiday. "I have always enjoyed working here and they have always seemed flexible," she said. But not about this. "Their attitude seems to be: if you don't turn up for work, you don't have a job."

A spokeswoman for Target said:

Target's opening time was carefully evaluated with our guests, team and the business in mind. Across the country, team member preferences were considered in creating our store staffing schedules. Thanksgiving weekend is one of the busiest of the year, and we appreciate our Target team's flexibility on this weekend and throughout the holiday season.

It's not hard to see why retailers are prepared to make their staff so unhappy. Last year, about 28.7 million people did their Black Friday shopping on Thanksgiving Day, according to the National Retail Federation, up from 22 million in 2010. The number is still less than a third of the roughly 86 million shoppers who took to stores on Black Friday itself, but it's growing, and likely to continue the trend this year.

Dan de Grandpre, CEO of Dealnews.com, a shopping deals tracking service, said Grey Thursday was here to stay. "Last year, Target and Walmart did it, and they were very successful. Stores that sat it out like JC Penney, for example, suffered," he said.

Retailers are aiming to capture as much of the money that shoppers have set aside for shopping as early as possible, said Grandpre. If the competition isn't open, they are potentially losing out.

"It's very tough for employees and it's a big decision to make, but if the consumers are willing to shop, the stores are willing to do it," said Grandpre. If stores benefit again this year, he expects even earlier opening times next year.

But Grandpre said that Dealnews's research showed 70% of the deals were available online, and that anyone expecting to avoid lines by arriving on Thanksgiving is likely to be disappointed. "Realistically, if the shop opens at 8pm on Thanksgiving, you are going to have to be there several hours earlier to make sure of getting the best deals," he said. With so many deals available online, Grandpre says he finds it hard to see the point.

Goebel, too, believes customers will eventually push back against Thanksgiving opening. "I truly believe that people would rather be spending the day with their families than spending it in line," she said.