Announcement: Upgrade to macOS Mojave
No one outside of Apple knows when (or even if) there'll be a new Mac OS. Apple will make announcements when they're ready. In the meantime, wait.
For the last several years, Apple has introducing a new operating system at the World Wide Developer's Conference in June, followed by a summer beta period, and release to users via the Mac App Store later in the Fall. The timing of the preceding events is entirely at Apple's discretion.
Question: When is the new macOS coming?