Apple hasn't officially announced when macOS Catalina will debut. The company's Danish website might have let the launch date slip, though, with MacRumors reporting yesterday that it referenced an October 4 release for the next major version of Apple's desktop operating system. That reference has since been removed.
macOS Catalina was revealed at WWDC in June alongside iOS 13, iPadOS 13, tvOS 13 and watchOS 6. Most of the announced changes were refinements to the operating system, but there are some more notable additions. A new feature called Sidecar turns a nearby iPad into a secondary display, for example, and iTunes was split into separate apps so people could actually enjoy interacting with various forms of media on their Macs again.
The release will also be accompanied by the Mac debut of Apple Arcade, the company's new game service that offers cross-platform access to its entire game library for $5 per month. This is where the Danish website might have let something slip. On a subtitle related to Apple Arcade, the site said that Apple Arcade will be available from the Mac App Store on October 4, which suggests that macOS Catalina would be released on the same day.
Right now the only thing known for sure is that Apple has said it plans to release macOS Catalina in October. More information about the update can be found on the company's website. The other software updates announced at WWDC--iOS 13, iPadOS 13, tvOS 13 and watchOS 6--have already been released.