It comes once every century (twice if you're using a 12 hour clock!): 11:11:11 on 11/11/11. In under 5 minutes I talk about the end of the First World War, the experience and value of the soldier, and what we can learn from them and from history.
P.S. The video was shot on 11 November, and posted here on the 12 @ 1:31 AM, but the time stamp has been changed to reflect the theme of the post.