Slightly as aggressive as how Microsoft rolled out Windows 10, Apple will be reminding and triggering an automatic download of macOS Sierra for compatible Macs. The users who have automatic downloads turned on and have enough storage space to install the download will get the update sometime “over the next week.” The update won’t instantly download without your permission, though. You will have to start it on your own—just like how Windows 10 updates were released. And if you aren’t ready to upgrade yet, you can just delete the installer or disable automatic downloads from the App Store. Plus, the update won’t install and be deleted if the Mac is starting to run low on storage.
Source: The Loop