It is his degree. He paid for half of it, so we let him decide when to walk and when to celebrate after graduation. Then they really celebrated when their respective student loans were paid off a couple years later. My two oldest will celebrate again when they complete their Master's degrees and their companies pay for it.
To answer the question directly..."When do you celebrate?".... my wife and I really celebrated when he (they) had a job ready after college, started in their respective careers, and moved into their own place. Clearly a bitter sweet occasion, however the eagle has to leave the nest at some point. 