Only Ubuntu published the Ubuntu 8.04 (Hardy Heron) release schedule
>> Ubuntu 8.04 (Hardy Heron) Release Dates
In addition to Ubuntu 6-months stable releases and the next version of ubuntu is 8.04 with Code Name "Hardy Heron" but this release will proudly wear the badge of Long Term Support (LTS) and be supported with security updates for five years on the server and three years on the desktop.
Hardy Heron was announced on Wednesday(29th August 2007) on the blog of Jono Bacon, the Ubuntu community manager for Canonical, the operating system's commercial sponsor.
>> Ubuntu 8.04 (Hardy Heron) Release Dates