I worked at Disneyland from October 1975 until January 1981. Thanksgiving and Christmas were always crowded then. I always worked those days, as the Union contract paid us double time.
Last time I was at the park was a random Wednesday, two February's ago. Place was deserted.
Worst day I ever remembered was July 4, 1976. Seem to recall the attendance that day was 72,000 (in those days gate entry count was posted the following day for staff to see). At the time 55,000 was considered an approximate sell out. The front gates typically would be closed for new guests, when the parking lot was full.
Park is much larger in size now, versus the 70's.