Trumpets were used for at least 6 different reasons in the Old Testament, and each one is relevant to the second coming of Christ …
 It is time to meet God for a very solemn occasion:
Exodus 19:16 at the giving of the 10 Commandments;
“On the morning of the third day, there was thunder and lightening, with a thick cloud over the mountain, and a very loud trumpet blast. Everyone in the camp trembled. Then Moses led the people out of the camp to meet with God, and they stood at the foot of the mountain. Mount Sinai was covered with smoke, because the Lord descended on it with fire.”
What an awesome picture!
The trumpet announces; ‘Come you people, it is time to meet God for a very solemn occasion!’
People may push God aside, but there will be a day when they will have to attend His awesome Presence.
A trumpet will be blown at the end of the world to announce it is time to meet the living God.
 It is time to meet together as a joyful community:
“Make two trumpets of hammered silver, and use them for calling the community together.”
Again, what a picture, it is for calling God′s people together, all the saints throughout history and the ones living at Christ′s return, a huge gathering of His people.
“For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God, and the dead in Christ will rise first.
After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air. And so we will be with the Lord for ever.” 1 Thessalonians 4:16-17 (ANIV)
 It is time to go into battle:
“When you go into battle .. against an enemy who is oppressing you, sound a blast on the trumpets”
At Christ′s second coming at the end of the world it will be the final battle against the forces of evil.
“And the devil, who deceived them, was thrown into the lake of burning sulphur, where the beast and the false prophet had been thrown. They will be tormented day and night for ever and ever.” Revelation 20:10 (ANIV)
 Trumpets in worship:
In Psalm 81 the trumpet assists in worship.
The sound of the tambourine, harp and lyre was the JOYFUL part of the worship time, but the sound of the trumpet was the SOLEMN announcement to balance that joyful worship with reverence and fear.
The trumpet at the last day, announces:
“Worship God you people, with reverence and fear.”
 Required to be blown over the sacrifices:
“You are to sound the trumpets over your burnt offerings, and fellowship offerings.” Numbers 10:10
Those offerings would have been brought with weakness and failings.
Jesus, our High Priest, will blow the trumpet over our duties and good deeds that we have done.
All our good deeds and labours done as a result of faith in Christ will be acceptable and pleasing to God.
 The day of release:
In Leviticus 25:8 it speaks of blowing the trumpet every 50 years, in the year of Jubilee.
It was the year of release, when any land that had been sold, was returned to it′s original owners, slaves were released, etc, etc.
All the believers will be released from the bondage of corruption, and will be given new resurrection bodies. Paul wrote:
“Listen I tell you a mystery; we will not all sleep, (that means die) but we will be changed – in a flash, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, the dead will be raised imperishable, and we will be changed. For the perishable must clothe itself with the imperishable, and the mortal with immortality.” (1 Corinthians 15:51)
That passage describes how believers will receive our new resurrection bodies.
“So if anyone tells you, ‘There he is, out in the desert,’ do not go out; or, ‘Here he is, in the inner rooms,’ do not believe it.
For as lightning that comes from the east is visible even in the west, so will be the coming of the Son of Man.
Wherever there is a carcass, there the vultures will gather. Immediately after the distress of those days ‘the sun will be darkened, and the moon will not give its light; the stars will fall from the sky, and the heavenly bodies will be shaken.’
At that time the sign of the Son of Man will appear in the sky, and all the nations of the earth will mourn.
They will see the Son of Man coming on the clouds of the sky, with power and great glory.
And he will send his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of the heavens to the other.
Now learn this lesson from the fig tree: As soon as its twigs get tender and its leaves come out, you know that summer is near. Even so, when you see all these things, you know that it is near, right at the door. I tell you the truth, this generation will certainly not pass away until all these things have happened.
Heaven and earth will pass away, but my words will never pass away.” Matthew 24:26-35 (NIV)