The format depends on who you want to celebrate and with whom. Internally focused: a team outing or party that celebrates the collaboration works well when team culture takes centre stage. Externally focused: a client dinner or exclusive drinks reception where you show clients and partners how far you've come.
Mixed: an internal moment during the day combined with an evening reception for clients. Whatever you choose: build in a moment of real recognition. Thank people by name. That's what people remember.