Your Mac will automatically download macOS Sierra starting today

Updates will be downloaded only when adequate storage is available and automatic downloads are enabled
Taking a page from Microsoft’s book, Apple has announced that its macOS, the latest iteration of Sierra, will be pushed as an automatic download. If automatic downloads are enabled on your Mac with enough free space, you’ll receive automatic updates next week, although the timing will be randomly chosen by Apple.
At first glance, this might seem like an aggressive move, similar to what Microsoft did with Windows 10. But Apple keeps it a little low, because the update is only downloaded but you do not install it without accepting the installation at the prompt. And if your system runs out of space while downloading the update, it shuts down and the file is deleted. Although this method is usually used to keep users up to date with the latest security features and bug fixes, it is intended to streamline the update process and bring a large portion of users to Apple’s latest operating system for the Mac.
If not now?
If you don’t want to download this update automatically, go to your Mac’s Settings app and click on the App Store. There, uncheck the option to download updates automatically. If and when you actually decide to get it, you can re-enable this option and wait for the popup, or you can go to Apple’s Mac Store software service, where you will see it posted as a first page download. Note that Sierra will only work on iMac or MacBook models from late 2009, 2010 or newer MacBook Pro, MacBook Air, Mac Mini or Mac Pro. For a more detailed list of requirements, see their upgrade page here.
A demo of Siri on MacOS
The Sierra update most notably brings Siri to MacOS, along with many more improvements like Optimized Storage, Universal Clipboard, and Proximity Unlock with Apple Watch. It is designed to bring macOS and iOS platforms closer together and will be a clear benefit for those who decide to update.