The Day of the Lord is in fact the coming of the Lord.
For this we say to you by the word of the Lord, that we who are alive and remain until the coming of the Lord will by no means precede those who are asleep. For the Lord Himself will descend from heaven with a shout, with the voice of an archangel, and with the trumpet of God. And the dead in Christ will rise first. Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And thus we shall always be with the Lord. Therefore comfort one another with these words. But concerning the times and the seasons, brethren, you have no need that I should write to you. For you yourselves know perfectly that the day of the Lord so comes as a thief in the night. 1 Thessalonians 4:15-5:2
Now, brethren, concerning the coming of our Lord Jesus Christ and our gathering together to Him, we ask you, not to be soon shaken in mind or troubled, either by spirit or by word or by letter, as if from us, as though the day of Christ had come. 2 Thessalonians 2:1-2
JLB
The Bible gives us many prophecies that certain things have to happen before Jesus returns. The two witnesses, mark of the Beast, wars, antichrist deeds like the abomination of desolation. When we see these things happening we know that 1260 days after the antichrist walks into the temple and declares himself God that Jesus will return. (Revelation and Daniel)..Here:
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In
Revelation 11 and
12, John mentions 1,260 days in two prophecies concerning another persecution of the Jews during the end times.
Daniel 8 speaks of 2,300 days in a prophecy concerning a persecution of the Jewish people during the
intertestamental period. The main differences between these two prophecies are 1) Daniel’s has been fulfilled, and John’s has not; and 2) Daniel predicts the actions of
Antiochus Epiphanes, and John predicts those of the
Antichrist.
The 1,260-day prophecy is found in two passages in Revelation. First,
Revelation 11:2–3 says,
“[The Gentiles] will trample on the holy city for 42 months. And I will appoint my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth.”
Then, as part of a
symbolic vision,
Revelation 12:6 says,
“The woman fled into the wilderness to a place prepared for her by God, where she might be taken care of for 1,260 days.”
The time period covered, 1,260 days, figures to 42 months, or 3 1/2 years. We believe this prophecy has not yet been fulfilled but will be during the end-times tribulation. The 42 months refers to the reign of the Antichrist, specifically, the last half (3 1/2 years) of the
seven-year tribulation. At the beginning of that time, the Antichrist will break his covenant with Israel and set up “
the abomination that causes desolation” (
Mark 13:14; cf.
Daniel 9:27)—an act that links the Antichrist to Antiochus Epiphanes, who similarly defiled the temple. The Antichrist will then turn his attention to the genocide of the Jews. During the persecution, Israel (the woman of
Revelation 12) will be protected by God in the wilderness. Also during that troubled time, God will send
two witnesses to perform miracles and proclaim the truth of Christ in the face of the Antichrist’s lies (
Revelation 11:5–6).
The detailed prophecies contained in God’s Word are part of what makes the Bible unique among religious texts. Our God can “make known the end from the beginning, from ancient times, what is still to come” (
Isaiah 46:10), and He has revealed significant events in the future, counting out the very days of those periods of time.../https://www.gotquestions.org/prophecy-1260-days-Revelation.html
So if we know when the 2nd coming will happen, why does Jesus tell us to be ready always at a moment, and for us to be watching?
Because it's in two visits. The church is gone by the time the tribulation starts.