The Ring is not open to the public all the time - it's used for all sorts of other things, like manufacturer test sessions, training schools, private hire days, etc. Indeed, the Ring is generally only open to the public on weekday evenings, a handful of weekends and the odd half-day here and there.

The 2008 calendar can be found on the official site.

If you're a member of the Ringers forum, you can click on the sessions to see who else will be there on any given day, and add your name to the list:

http://forum.ringersforum.com/mycalendar.php