Sign of the Son of Men
There is debate among Bible teachers as to what is the 'sign of the Son of man'. Most think the sign of the Cross will appear in the sky just prior to His appearing. However, though the symbol of the Cross is common among Believers today, it was not necessarily so in the first century. It may not be what Jesus had in mind when He gave this prophecy.
Scripture seems to indicate that the scars inflicted during Jesus' Crucifixion is the more probable 'sign' mentioned here. They are the definitive symbol of the world's rejection of Christ as Messiah and Savior, and will cause the non-Believers to 'mourn' and regret their actions ... but it will be too late. For them, the Judge is come and their punishment is certain on that terrible day of the LORD's wrath.