Planet13 - 10:05 pm on Aug 23, 2013 (gmt 0)
"...the pages all have proper content on them, they are not thin -- they are just out of date (the date of the event will still be shown on the page)."
Would a person be HAPPY to find it in the google SERPs, or would they be bummed?
Is the date of the event in the title so they would know BEFORE CLICKING ON IT that it was for a past / expired event?
For the past events that you do have still in the index, what is the bounce rate like? Are the coming to the page and then leaving? Or are they checking out other pages on your site?