Eh. Berryz have their celebration is March 3rd - the day they had their major debut.
They were formed in January - I don't know if they had any fanclub events around their official formation anniversary though.
More than anything, /years/ count though. Berryz have their "major debut 10th year" this year and they're doing things the whole year. It'd make more sense to the public to go by major debut since that's what they see -- they don't see fanclub events and whatever since that's only fanclub members.
But, however they do it, if they do a "10th year since formation" thing next year for Cute, that's great. If not? Then too bad. I'm sure they'll get Budoukan either way.