30Then will appear in heaven the sign of the Son of Man, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory.
The sign of the Son of Man is either a general sign/standard for the coming of the eschaton,1 or the appearance of the Son of Man (Jesus) himself.2
30 Then will appear in heaven the sign of the Son of Man, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory.