D&C 88

After prophecying of several signs of the times, the Lord then said to His apostles -

"...then shall appear the sign of the Son of man in heaven..." (New Testament | Matthew 24:30)

In modern day, we read of the same prophecy -

"And angels shall fly through the midst of heaven, crying with a loud voice, sounding the trump of God, saying: Prepare ye, prepare ye, O inhabitants of the earth; for the judgment of our God is come. Behold, and lo, the Bridegroom cometh; go ye out to meet him.
93 And immediately there shall appear a great sign in heaven, and all people shall see it together." (Doctrine and Covenants | Section 88:92 - 93)

It is not clear what this great sign will be, but it will be such that all people shall see it together.

Amos said -

"Surely the Lord GOD will do nothing, but he revealeth his secret unto his servants the prophets." (Old Testament | Amos 3:7)

In commentary on the Doctrine and Covenants we read -

"It may be gathered from this that when the sign appears, God will make its meaning known to the Prophet, Seer and Revelator who at that time may be at the head of the Church, and through him to His people and the world in general." (Smith and Sjodahl, DCC, p. 560.)

It will be a marvelous day when the Lord's prophet declares the sign of "the sign of the Son of Man in heaven".