Mark Gurman, writing for 9to5Mac:

Apple currently plans to use its next major release of the Mac operating system, known as OS X 10.12, this fall to continue to expand Siri across its product lines. Last year, Apple implemented Siri as cornerstone features of both the Apple Watch and new Apple TV, and for 2016, Siri is planned to finally make its way to the Mac.

Apple had been testing versions of OS X internally with Siri integration since at least 2012, but sources now say that Apple has a clear vision for Siri on the Mac along with a polished user-interface and is nearly ready to launch the feature publicly. Apple is expected to introduce OS X 10.12 in June at its annual Worldwide Developers Conference.

Fantastic! I use Siri all the time, especially to create reminders and add appointments to my calendar. I would absolutely love the ability to tap a command key and use Siri to do the same thing on my Mac. This has long been a missing feature for me.