Catholic Christians have always believed that Jesus Christ would come back to close the current period of human history in earth. The time when Jesus will return is given many names: the Day of the Lord, the Parousia, the end time, and the Second Coming of Christ.
The Bible describes the events of Jesus' return in apocalyptic images.
And then they will see 'the Son of Man coming in the clouds' with great power and glory, and then he will send out the angels and gather (his) elect from the four winds, from the end of the earth to the end of the sky.
For the Son of Man will come with his angels in his Father's glory, and then he will repay everyone according to his conduct.
They (two men dressed in white) said, "Men of Galilee, why are you standing there looking at the sky? This Jesus who has been taken up from you into heaven will return in the same way as you have seen him going into heaven."
1 Thess 4:16-18
For the Lord himself, with a word of command, with the voice of an archangel and with the trumpet of God, will come down from heaven, and the dead in Christ will rise first. Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air. Thus we shall always be with the Lord. Therefore, console one another with these words.
The parousia will be unmistakable because it will be accompanied by unprecedented signs.
For just as lightning comes from the east and is seen as far as the west, so will the coming of the Son of Man be.
Some signs are general events concerning the evangelization of the world.
And this gospel of the kingdom will be preached throughout the world as a witness to all nations, and then the end will come.
Other signs are more proximate. Mark (Chapter 13), Matthew (Chapter 24), and Luke (Chapter 21) all describe the unmistakable signs with apocalyptic images.
2 Tim 4:1-2
I charge you in the presence of God and of Christ Jesus, who will judge the living and the dead, and by his appearing and his kingly power: proclaim the word ...
2 Tim 3:1-5
But understand this: there will be terrifying times in the last days. People will be self-centered and lovers of money, proud, haughty, abusive, disobedient to their parents, ungrateful, irreligious, callous, implacable, slanderous, licentious, brutal, hating what is good, traitors, reckless, conceited, lovers of pleasure rather than lovers of God, as they make a pretense of religion but deny its power. Reject them.
No one knows exactly when this will occur.
(Jesus began his discourse ...) "But of that day or hour, no one knows, neither the angels in heaven, nor the Son, but only the Father."
1 Thess 5:2
For you yourselves know very well that the day of the Lord will come like a thief at night.
2 Pet 3:10
But the day of the Lord will come like a thief, and then the heavens will pass away with a mighty roar and the elements will be dissolved by fire, and the earth and everything done on it will be found out.
The Catholic Church teaches that we should avoid pointless speculations about the time, the details of the signs, the nature of the difficulties, etc. The Church focuses instead on the need for living the Gospel so as to be prepared for the parousia whenever it happens.
(Jesus began his discourse ...) "Be watchful! Be alert! You do not know when the time will come. ... Watch, therefore; you do not know when the lord of the house is coming, whether in the evening, or at midnight, or at cockcrow, or in the morning. May he not come suddenly and find you sleeping. What I say to you, I say to all: 'Watch!'"
1 Pet 1:13-16
Therefore, gird up the loins of your mind, live soberly, and set your hopes completely on the grace to be brought to you at the revelation of Jesus Christ. Like obedient children, do not act in compliance with the desires of your former ignorance but, as he who called you is holy, be holy yourselves in every aspect of your conduct, for it is written, "Be holy because I (am) holy."