While the old stories of werewolves changing during the full moon aren't really true, they have some basis in fact.
The difficulty to frenzy on a new moon is 8; this goes down by phase, with the difficulty on a full moon being 4.
The difficulty drops by one if the Garou is in Crinos, or if it is their auspice moon. (These are not cumulative modifiers.)
Aspen has coded an awfully useful frenzy checker, which can be seen (and used!) at http://web.garoumush.org/tools/cgi/frenzy.cgi.