Sundays are usually not packed. Saturday's are definitely the busiest, followed the Fridays, and then Thursdays and Sundays are usually the slowest.
And this past saturday (1st post-Freaky Preview weekend) usually seems to be the slowest Sat. of the event.
But yeah, it is definitely best to get there early. I remember when I attended my first HOS in 2003, it was a Thursday, and I was overwhelmed by how busy the parking lot and front gates were (I arrived around opening). It took me awhile to get in the park, but once I did, I was shocked by how empty the park was.