I was there in 2006, not intentionally, had no idea there was such a thing. We live in Northern California and go down there 3-5 times a year and that was our first annual pass trip. It was by far the most crowded Ive seen the park. I remember just trying to get lunch, the line for the Hungy Bear was over 30 minutes long and rides were ridiculous.
Anyhow, I was standing next to a gentleman who was there for the GD celebration and he was saying how that or the year before might of been the first year that the celebration was during the day, normally it would be a night like a Grad Night deal which he thought was much better because as he said, this is just too crowded.
Because of the enormous crowds we experienced that first year, we tend to try and not go down that weekend.