(CBSNews) Amazon's annual "Prime Day" is July 11. This year, however, the promotion will extend to 30 hours.

Amazon (AMZN) will offer discounts and other deals in an effort to boost sales during the slower summer months. The deals are open only to members of Amazon's $99-a-year Prime loyalty program, so Amazon typically gets more sign-ups, too.

While Amazon has claimed success, there have been grumblings that Prime Day deals are unimpressive or involve older models. Last year, Benchmark analyst Daniel Kurnos called Prime Day an example of "how to make clearance look good." After receiving criticism in the first year of the event, in 2015, Amazon vowed to improve the number and quality of bargains. 

Walmart (WMT), which recently unveiled similar service to Prime called ShippingPass.com, announced on Monday that it would offer free shipping and discounts on selected merchandise all week. Membership to ShippingPass, which like Amazon Prime offers free two-day shipping, is priced at $49 -- $50 less than the $99 annual fee that consumers pay for the better-known Amazon service.

For Amazon, the promotion helps lure more consumers into signing up for the Prime service, which has provided a huge payoff to the business. While Amazon doesn't disclose details about its Prime membership, Wall Street analysts have estimate that Amazon has about 65 million or so Prime customers in the U.S. On top of that, Prime members make about 150 percent more purchases than nonprime members, by one analyst's estimate. 

This year's promotion will start at 9 p.m. ET on July 10 and end at 3 a.m. ET July 12. It used to run for 24 hours. It will be available in 13 countries, including the U.S., U.K., Germany, Japan and China.