I didn’t know there were so many Xmas flash mobs! Never happens here. :-(
The only one I saw before this was in a mall by Santa and they brought in the holy family after much singing. I think I found it on YT but cannot link on iPad.
Wow! I belong to an enormous church in McLean, Virginia and they have a cafeteria. They did this last year, but with Christmas carols...I was never more surprised and delighted.