The Return of Jesus.

AND JESUS SAID

And then at last, the sign of the coming of the Son of Man will appear in the heavens, and there will be deep mourning among all the nations of the earth. And they will see the Son of Man arrive on the clouds of heaven with power and great glory. And he will send forth his angels with the sound of a mighty trumpet blast, and they will gather his chosen ones from the farthest ends of the earth and heaven.”  Matthew 24:30-31

The nations of the earth will mourn because unbelievers will suddenly realize that they have chosen the wrong side. Everything they have scoffed about will be happening and it will be too late for them.