Kubuntu Gutsy (Canllaith.org)

April and October were chosen because they're more or less equidistant between the summer and
winter holidays.  Even if Gnome went a full year without a release, Ubuntu would still follow
their six month cycle like clockwork.

That said, I'm sure it's no coincidence that Ubuntu tweaks their release date to consistently
fall a week or two later than Gnome.  :)