Kjell, even though you put the question out there as simply as possible, I am not sure I get it. Do you mean how do we commemorate and get into the feeling celebrating, or what do we drink and eat, or what do we do to be merry?
If I picture a group of people getting together at a restaurant for a work goodbye party, they all start out in the same way; they order something and sit there and talk (and wait till their orders come out). The celebrating has already started. Once the drinks come out, the celebrating gets colorized or modified by the drinks and in some cases the celebrating has actually stopped and something else has come about (how much of the purpose for being there is still present, in other words, once the alcoholic does its thing, especially for someone who has a "relationship" with alcohol).
I suppose a husband can drink while waiting for a baby to be born, but I can't imagine that being allowed while in a hospital room. Celebrating is experiencing a happy event, and the natural thing to do is to interact and enjoy the moment. If alcohol is present for some people, well, they are adding that to the mix.
Even during my "using" state, I was aware of how to celebrate without alcohol. I would abstain while others drank (or didn't). (I drank later, by myself and celebrated nothing, but that is a different story.) Now that I don't drink anymore, I can still somethign being celebrated. I can be goofy without being drugged up or I can laugh without being drunk, etc, etc. Not really sure where to go with this question, but I gave it a try.