A “flash mob” is a seemingly spontaneous event involving lots of people. They happen with no warning or advance notice and at the end of a predetermined amount of time everyone involved just resumes what they were doing.
Can you imagine being at the food court at Fox Run Mall in Portsmouth when seemingly random people stand up and start singing the Hallelujah Chorus? The action starts about 30 seconds into the video: a moving performance.
Very nice and an inspiring reminder of the true meaning of Christmas.
Best wishes to Portsmouth, New Hampshire and all the world for peace on earth and good will towards all men.