Get ready for the online sales wars.

After Amazon announced plans last week for a day of discounts Wednesday to rival Black Friday, Walmart is jumping into the fray.

Fernando Madeira, president and CEO of Walmart.com, said in a blog post on Monday that Walmart (WMT) will be launching its own July 15th sales extravaganza online that will offer "thousands of great deals" and "special atomic deals."

After Wednesday, sales are set to continue for 90 days.

Walmart has also cut its $50 minimum purchase for free shipping to $35 -- the same as Amazon's free shipping option.

To celebrate its 20th anniversary, Amazon is planning to offer sales as often as every 10 minutes and the company said the day would be "filled with more deals than Black Friday."

Walmart's blog post was clearly aimed at rival Amazon (AMZN), with a banner flashing "No Admission Fee. You shouldn't have to pay $100 to get great deals."

"The idea of asking customers to pay extra in order to save money just doesn't add up to us," Madeira said.

That's a reference to Amazon's regular $99 membership fee for Amazon Prime.

Amazon, however, is not going down without a fight.

"We've heard some retailers are charging higher prices for items in their physical stores than they than do for the same items online," Greg Greeley, Amazon Prime global VP, said in a statement to CNNMoney. He added that having costlier in-store products "doesn't add up," and reinforces the idea that "you're usually better off shopping online."

Walmart has only recently launched its own pilot unlimited shipping service this year to rival Amazon's decade-old Prime shipping service.

"If you are one of the tens of millions of Prime members, you're all set," said Greeley.

Amazon is offering free 30-day Prime memberships for those who want to take advantage of the expected huge sales on Prime Day.