According to the latest rumors, Apple is planning to release its new OS X Yosemite in late October, a month after iOS 8. Specifically, it plans to seed a final Developer Preview on 29 September and a Golden Master build around 10 October.
Rumors also suggest that alongside OS X Yosemite, Apple will also launch brand new Macs with high resolution displays. Specifically, word on the street is that Apple is "finishing up work on both a smaller MacBook with a high-resolution display and a new desktop computer, either an iMac or a standalone monitor, with a 4K resolution screen."
This makes sense since the new Macs will be able to complement the launch of OS X Yosemite, which, by the way, will be available as a free download on the Mac App Store.