It's similar to what Microsoft is doing with its forthcoming Windows 8 system. That system, to be released Oct. 26, will bring the look and user interface of Windows Phone to PCs.
Mountain Lion costs $20 and is sold only as a download from the Apple App Store. Only computers running the two most recent versions of Mac OS, Lion and Snow Leopard, can be upgraded.
Macs bought on or after June 11 can be upgraded for free.
Apple previously announced Mountain Lion's features, but it hadn't disclosed the exact availability date until Tuesday's release of its earnings for the April-June quarter.
Among the features of Mountain Lion:
— The new software has better integration with social networks such as Facebook Inc.'s. It has built-in features to facilitate sharing on Facebook, Twitter, Flickr and other services. For instance, you'll get notifications when you get a message or a mention in a Facebook or Twitter post. You need to sign in only once, and you can share directly from other apps you are using.
— Power Nap keeps your Mac updated even while it's in power-saving "sleep" mode. It gets your email messages, backs up your files and downloads software updates automatically. It works with recent MacBook Air computers and the higher-end MacBook Pro model, the one with the sharper, "Retina" display.
— A new Messages app, copied from Apple's mobile operating system, replaces iChat. It allows you to send messages to other Apple users, whether that person is on a Mac or an Apple mobile device.
— Mountain Lion is integrated with iCloud, the new Internet storage service designed for the mobile devices.
— The software brings dictation to Macs, essentially allowing the computer to type as you talk.
— Game Center stores high game scores and helps users find opponents on both Macs and Apple mobile devices.