I've found that I enjoy events as these much more when I participate than when I just consume passively, so when there was a call for volunteers I jumped in.
The parade consisted of historical trams and a few new ones from other cities. Naturally they all lacked the VETAG-equipment that regular trams carry to automatically set the switches in the right position, therefore the GVB turned the automatic system off, and all the switches had to be changed by hand. That's where the volunteers were needed, and that's what I had to get out of bed for at 7.15 on a beautiful sunny Sunday morning.
In short, I greatly enjoyed the day, and so did 2000 visitors. For an impression of the trams, see these galleries: