The news comes from HP's Vice President of Product Management for the Consumer PC and Solutions, Mike Nash, in a blog post:
"As we get closer to Windows 10 availability, I want to clearly explain how HP will deliver new devices with Windows 10 pre-installed to customers. Right now, a customer can buy select PCs with Windows 10 directly from HP.com and it will ship on July 28th. We are offering free next-day shipping so customers will receive the device on July 29th. Customers will start seeing HP devices running Windows 10 in-store at select retailers beginning August 2nd."
This news means that HP will actually be one day ahead of rival Dell, as it announced a few weeks ago it would begin shipments of some Windows 10 PCs on July 29, with next-day delivery on July 30.