Donald Knuth's Annual Christmas Tree Lecture - Free Live Lecture and Webinar

That was the page for the live stream for this past Monday. If you look at it now, you see:

Thank you for your interest in this course. We regret that the course you selected is not currently open for enrollment.

At some future date (hopefully soon), the video should be posted to http://scpd.stanford.edu/knuth/index.jsp, where the archive of Knuth's previous lectures lives.