To a Hebrew follower of Jesus that means He is returning on the feast of Trumpets.
The Feast of Trumpets began on the first day (at the new moon) of the seventh month.
You had to be “watching” for the appearance of the new moon because no one knew the day or hour it would appear to mark first day of the new month and the feast of trumpets.
Paul understood this as a student and follower of Jesus Christ.
Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed— in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed.
1 Corinthians 15:51-52
We as Christians, who study to show ourselves approved unto God, knows that the Day of the Lord will not come as a thief in the night for us.
But you, brethren, are not in darkness, so that this Day should overtake you as a thief.
1 Thessalonians 5:4
JPT