What does it mean for all the nations of the earth to mourn?
Matthew 24:30 (ESV)
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 mourning of nations has roots in Zechariah 12:10–14 where it refers to mourning for sin. In the mouth of Jesus, it refers to mourning for the judgment to come, a judgment which is spelled out in Revelation 19:11–21.1 There will be no opportunity for repentance at this time.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.