Mt 24:30-31, 1 Thess 4:16, Rev 11:15, Dan 7:13-14
God made the Israelites blow a trumpet on the day of the Feast of Trumpets. This is to inform the people of Israel that a new day of God has come. (Lev 23:24)
Lev 23:24 Speak unto the children of Israel, saying, In the seventh month, in the first day of the month, shall ye have a sabbath, a memorial of blowing of trumpets, an holy convocation.
In the Old Testament, it was foretold that Christ would come again in the clouds of heaven. (Dan 7:13-14)
Dan 7:13 I saw in the night visions, and, behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him. 14 And there was given him dominion, and glory, and a kingdom, that all people, nations, and languages, should serve him: his dominion is an everlasting dominion, which shall not pass away, and his kingdom that which shall not be destroyed.
Jesus, the Christ, will descend from heaven with the sound of the trumpet of God. (Matthew 24:30-31, 1 Thessalonians 4:16, Revelation 11:15)
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 in the clouds of heaven with power and great glory. 31 And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other.
1 Thessalonians 4:16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
Revelation 11:15 And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.