In the seventh month on the first of the month you shall have a rest, a reminder by blowing of trumpets, a holy convocation. You shall not do any laborious work, but you shall present an offering by fire near to Yahweh.
(Leviticus 23:24-25 LSB)
Numbers 29:1 And in the seventh month, on the first day of the month, ye shall have a holy convocation; ye shall do no servile work: it is a day of blowing the trumpets unto you.
When the trumpet sounds every Jew drops his work, and come in, for worship. On this day the trumpet is blowing 99 times, and then one more time. This last blow, the 100th time, is the last trump.
I 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. Most bible scholars believe this points to the time of the Rapture for various reasons.