The shortest holiday shopping season since 2002 has many national retailers opening their doors today, Thanksgiving Day, in a bid for more than $600 billion in annual consumer spending, experts said.

This year’s Thanksgiving falls as late as the calendar possibly allows, leaving just 27 days until Christmas, compared to 33 days last year. Consumers will have four, not five, full weekends this year for gift-buying in that time frame.

“The more time they can extend the shopping season the better for the retailers to make money,” said Serdar Durmusoglu, a University of Dayton associate professor of marketing.

Retail experts said the weak economy in the wake of October’s partial government shutdown, as well as increased competition from online retailers, also are helping to further shift the start of the holiday shopping season from its traditional date on “Black Friday.”

In recent years, a number of major retailers such as Wal-Mart, Target and Kohl’s started opening at midnight on Black Friday to give consumers an early start on holiday deals. Last year, Wal-Mart, Sears and Toys ‘R Us moved up their openings four hours earlier to 8 p.m. on Thanksgiving.

This year, Cincinnati-based Macy’s will open on Thanksgiving for the first time in the company’s 155-year history. Kmart stores will start offering Black Friday deals at 6 a.m. today.

Macy’s officials attributed the trend to consumer demand and being consistent with their competitors.

An American Express survey found that 27 percent of shoppers want to get their holiday buying done by December, up from 24 percent last year.

Despite public concerns about the increasing commercialization of a traditional family holiday, Thanksgiving is fast becoming the day for retailers to start their big holiday push for profits.

Last year, more than 35 million American visited retailers’ stores and websites on Thanksgiving, up 20 percent from 29 million in 2011, according to the National Retail Federation.

Online spending last year on Thanksgiving Day jumped 32 percent from 2011, according to data from marketing analytics firm comScore. During the past five years, online spending on Thanksgiving has increased 132 percent.

“Now more physical stores are open,” allowing shoppers to inspect items and try on clothing before purchasing, Durmusoglu said.

Black Friday takes its name from an accounting term that refers to profit being indicated in black ink and losses in red, according to Durmusoglu. “A majority of retailers, they actually become profitable at the end of the year, especially … right after the Black Friday sales,” he said.

The November-December holiday shopping season can represent as much as 20 to 40 percent of annual sales for some retailers, according to the National Retail Federation.

The retail federation is forecasting holiday season sales of $602.1 billion, an increase of 3.9 percent over 2012’s sales growth. Last year, holiday sales increased 3.5 percent to $579.5 billion. The forecast is higher than the 10-year average holiday sales growth of 3.3 percent.

Ohio consumers are expected to spend nearly $15 billion during this year’s holiday shopping season, a 3.5 increase from 2012, according to a University of Cincinnati economic forecast released this week.

Online holiday sales this year are forecast to grow from 13 to 15 percent over 2012 to as much as $82 billion, according to Shop.org, the retail federation’s digital commerce division.

Retailers are expected to hire between 720,000 and 780,000 seasonal workers this holiday season, which is in line with the 720,500 seasonal jobs added in 2012, federation officials said.

Many retail employees likely will be working today when stores open their doors to throngs of holiday shoppers, instead of being home with their families. “This has been an ongoing issue for the last few years,” Durmusoglu said.

Some retailers are offering employees perks and extra pay for working the Thanksgiving holiday. For example, Wal-Mart said an estimated 1 million employees working today will receive extra “holiday” pay, a turkey dinner and 25 percent off a future purchase. Macy’s allowed workers to volunteer for the holiday shifts they prefer.

Durmusoglu said it is unlikely retailers will ever go back to being closed on Thanksgiving, even with a longer holiday shopping season. “Once you give something, you can’t take it back easily from consumers. There might be a backlash,” he said.