Its driven by a pocket query, which is set to generate daily with the following criteria:
1. Selected cache types: Event Cache, CITO Event, Mega-Event Cache
2. "Is Active"
3. Within Oregon
No other restrictions are placed on the query. So any event cache, which is active, and is within what geocaching.com considers to be Oregon (I'm not sure, for example, if an event were held on the Snake River, if it would count).
Expired events will currently only go away after they have been marked Inactive by the cache owners. I'll probably alter this behavior to show up to X expired caches at a time, so that we don't end up with a huge list.

If an event cache is scheduled far in the future, 2012 for example, it'll still show up as long as it is marked as "Active".
The script will start updating daily automatically tomorrow, once everything has been moved to the new server. I uploaded a test run to the current server on Friday, so that's why it hasn't updated for today.