"Speak unto the children of Israel: In the 7th month [Tishri] on the first day of the month ye shall have a day of rest, heralded by the sound of a trumpet, a holy convocation." Leviticus 23:24.
A double Feast because it is at the same time a New Moon Day, but also a separate Feast at the same time! So a Great Sabbath. A Feast that is all about renewal and announcing the Second Coming of Christ and the new life that will come with it.
The Bible speaks of that moment with the sound of a trumpet when God sends Christ to us again together with the angels to have His Children brought to Him in Heaven:
"But don't we believe that Jesus died and rose from the dead? Then it follows that God will take to Himself those who died as Christians, together with Jesus. We keep a word from the Lord when we say to you: we who are still alive until the coming of the Lord have no precedence over the dead. For when the sign is given, the archangel lifts up his voice and God's trumpet sounds, the Lord himself will descend from heaven. First, then, the dead Christians rise, then we who are still alive are caught up with the resurrected in the clouds in the air to meet the Lord. Then we will be with the Lord forever." 1 Thessalonians 4:14-18.
So no church theology of "instant heaven or hell" at death, but revival of Christians after the Second Coming of Christ and the angels sent by God YHWH. 1 Resurrection to life + going with Christ and the angels, and 1 resurrection to Judgment, execution and eternal death a 1,000 years later.
Trumpet Day is the announcement of Christ's Second Coming. Once the announcement is made, you want to be prepared for the actual 2nd Coming of Christ via fasting on the Day of Atonement!