I never saw the birthday cake ceremony, but I heard about it a number of times.
Since I was strictly a domestic flyer in those days, the closest I ever got to International service on PanAm was the early afternoon flight from IAH to JFK. It was the last leg of the around the world flight and had the big seats in FC, champagne, high end meals, and great service. It was well worth forgetting about running for a late flight after work, or making sure to get done early in order to get to IAH for that flight, which usually left about 130-2PM.