The different branches (xubuntu for example): are they released the same day as the main branch? IE will xubuntu 12.04 come out the same day as Ubuntu 12.04, or is there a lag? Is there an official policy posted somewhere?
The closest thing to "official" that you can get is "when its ready."
Generally speaking, they are released on the same day, however this is not set in stone, because there might be a release blocker that pops in late in the cycle.
For example, Xubuntu 11.10 was released on 13 October 2011, as was Kubuntu and normal Ubuntu.
10.04 Kubuntu, Xubuntu and Ubuntu were also released on the same day (29 April 2010).
As of right now, it appears that official derivatives will be released on the same day as "normal" Ubuntu 12.04.
As per GridCube on the #xubuntu irc channel: