I just returned from my younger daughter's fifth grade graduation. (For those of you without children, sixth grade is now part of middle school.) It was lovely, and reminded me of how important it is for us to have rituals or ceremonies for the ends of significant work.
If you're scheduling a project, schedule in the retrospective and the celebration at the beginning. I schedule both at the beginning, so that everyone's expectations are set (management and the project team). We're going to learn about this project in a formal way (retrospective), and we're going to ceremonialize the end.
I've been on projects where we didn't have a retrospective. We made all the same stupid mistakes again.
I've been on projects where the last day was just the last day. No special acknowledgement. No appreciations. No lunch. No nuthin. Big let down.
Make part of your project completion a retrospective and some sort of celebration. You and your project team will thank you.