Celebrate is only ever used as a Z-Move, if powered up by Normalium Z it raises all stats at once.
The Pokémon congratulates you on your special day. No battle effect.
When a Pokémon is holding Normalium Z and uses its Z-Power, Celebrate turns into Z-Celebrate and raises all stats one stage each, in addition to its usual effect as above.
Other than that it's a useless move and even with Normalium Z it's still a gimmick.