Sorry for shaky phone cam work! Audio is from the car and is kind of low until the doors open...
Discovered this easter egg was finally in the firmware in 2.50.15 a couple of days ago... but my X was at the service center getting its first annual service done, so I couldn't mess with it until tonight! So, while I'm pretty sure I'm the first to discover that it was in the firmware (announced it on my IRC channel days ago), I wasn't the first to video it working. Oh well! Still fun!
Tesla showed off a version of this last year, but only released the code to customer cars this week.
It was actually broken in 2.50.15. The code "holiday" was used to activate it, but it would stop part way through. Then 2.50.114 came out and completed it and changed the code to "modelxmas".