A project should not only start with an event (see 'Kick-off Retreat' and 'Kick-off Meeting') but its end should also be marked in some way. This is important in order to 'let the project go' and be ready for a new start.
At the end of the project its structure is dissolved and the team members' professional future may possibly lead them in different directions. Some collaborators may leave the institution, others will continue in new projects with new colleagues.
Everybody has invested much energy into creating good teamwork and relationships that might continue after the end of the project have been created. The dissolution of a team is always a sort of farewell that should be marked appropriately, e.g. by passing the project in review.
The best way to do this is to organize a (budgeted) final get-together that marks the end of the project for all concerned, including to the institution itself. Invite all participants of the kick-off meeting and everybody else who was actively involved in the project.
You neglect the active dissolution of the project's structure and the marking of the project's end because:
If you dissolve the structure of a project actively you provide an opportunity for feedback and therefore are able to express the deserved appreciation of the team's work; you also help avoid project fatigue. As a result the institution will have less fluctuation in personnel and can retain qualified people; the project's partners will be motivated for further collaboration.
If you celebrate the end of your project:
Possible ways for marking the end of a project: