A.T. Robertson Commentary Matthew 24:30

"and then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming on the clouds of heaven with power and great glory." — Matthew 24:30 (ASV)

The sign of the Son of Man in heaven (το σημειον του υιου του ανθρωπου εν ουρανω). Many theories have been suggested like the cross in the sky, etc. Bruce sees a reference to Da 7:13 "one like the Son of man" and holds that Christ himself is the sign in question (the genitive of apposition). This is certainly possible. It is confirmed by the rest of the verse: "They shall see the Son of man coming." See Mt 16:27; 26:64. The Jews had repeatedly asked for such a sign (Broadus) as in Mt 12:38; 16:1; John 2:18.