Ubuntu follows a 6-month release cycle, denoted by the version number, so ...

7.04 = April 07
7.10 = October 07

The next will be 8.04, as noted above, in April ...

There has been 1 exception to this rule - there was no 6.04 rather,
due to a decision to make that release an LTS (Long Term Support)
release, it was put back a couple of months to ensure it's stability -
this was 6.06.