What is the sign?
I believe it is the Son of Man Himself, because of the grammar of the verse. What is your thought?
Much love!
We do not know exactly what that sign is but it is the sixth seal. I can surmise that it is the sky rolling up like a scroll, which is the one thing that differs from the Matthew 24:29-30 account and the Revelation 6:12-14. Read the two passages as parallel accounts to see this, Below.
And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood; And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.
And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places. (Revelation 6:12-14)
Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
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 in the clouds of heaven with power and great glory. (Matthew 24:29-30)