A few years ago I created a public google calendar for agile conferences. These days it is maintained by 47 people. If you think your conference is missing,please contact me.
Update: This is the direct link to add it to your google calendars: http://www.google.com/calendar/hosted/hanoulle.be/embed?src=hanoulle.be_vhhuilrdov5hiodkhauquvp7eg%40group.calendar.google.com&ctz=Europe/Brussels
Also check out the Google Agile events calendar