It is the best and worst day of your life.
You are thrilled by the success that it shows, that justified the loss of sleep, missed family events, forgone social life, worry, fear and all other emotions. But it is your baby. When you walk in the next day (if you get to stay, which is not a given) it is no longer yours and you know the idea will change with the new owners. No matter what you are told, it will change and conform to what they want, not what you want. The day after closing is the first day of you being gone, if not physically, at least mentally.