Some 80% of major retailers will send e-mail marketing messages to consumers on Thanksgiving Day this year, according to Chad White, research director at Responsys. That compares with 76% last year, 60% in 2010 and 45% in 2009. The e-mail services provider bases its figures on e-mails sent to subscribers by the largest 100 e-retailers as measured by revenue.

White says that Thanksgiving Day stands as the third busiest day for e-mail behind Black Friday—the day after Thanksgiving—and Cyber Monday—the first Monday after the holiday. “Thanksgiving Day was traditionally a family day, but now it’s in play for sales,” White says, adding that the recent trend of big-box retailers opening on the holiday has helped to fuel the e-mail boom.

What this means, according to Responsys, is that “Thanksgiving has replaced Black Friday as the unofficial kick-off the holiday shopping season.” In fact, last year, online shoppers spent $479 million on Thanksgiving Day, up 18% from 2010, according to comScore Inc.

The e-mail marketing messages that retailers will send this Thanksgiving will fall into two general camps, White says: Messages promoting Thanksgiving-only sales, and others teasing Black Friday sales, or offering those sales at least a few hours early. “One camp is trying to keep Black Friday pure, while another is trying to use the (allure) of Black Friday for Thanksgiving,” he says. A much smaller camp involves those retailers telling consumers that stores will remain closed on the holiday—in essence, retailers making “social statements” about the importance of family, White says.

Consumers’ embrace of smartphones and tablets also has led to more retail e-mail messages hitting inboxes on that last Thursday in November, he says. After all, what better way to reach shoppers who are on the road,  away from their work computers and personal laptops—and perhaps eager for distraction? “Mobile has really enabled a lot of e-commerce on Thanksgiving that really wasn’t there in the past,” White says. Retailers also will use the days before Thanksgiving to remind consumers, via e-mails, of mobile efforts undertaken by merchants, he adds—that is, preparing the ground for mobile shopping during the holidays.

White doubts that Thanksgiving will overtake Black Friday and Cyber Monday as the busiest retail e-mail day, though the holiday will certainly become more crucial for e-mail marketing. He says another holiday is also becoming more important for e-mail marketing: Christmas. That’s when consumers and their mobile devices are again away from home, and when retailers are eager to encourage those shoppers to buy goods with their newly unwrapped gift cards.