The video of Berklee College of Music students performing “O Holy Night” flash-mob style at the Museum of Fine Arts over the weekend has officially gone viral. Footage of the musical event was posted Wednesday, and by Thursday afternoon, it had been viewed more than 63,000 times. About 50 performers participated in the song, including soloist Mark Joseph. “I just told myself, ‘Mark, don’t stress,’ ” Joseph said of the performance. “ ‘Yeah, you’ve got one chance but whatever, sing the song and do your best to get the message across. And keep it simple.’ ” Students Evan Chapman and Josh Knowles came up with the flash mob idea a few months ago. The pair were brainstorming ways to share music outside of Berklee’s community, said Chapman, who was scheduled to graduate on Thursday. “Three weeks before the flash mob I was visiting the MFA and was taken aback by the room and was inspired by the space and felt it would be a perfect location to share one of my arrangements,” he said.