The Catechism in Questions & Answers

553. What follows from the fact that no man knows the moment in time of Christ’s return?

Since no man knows the moment in time of Christ’s return, the believer is called upon to remain prepared for this event every day. The Son of God summarises this into the admonition: “Watch therefore, for you do not know what hour your Lord is coming” (Matthew 24: 42). Jesus also made it clear in parables that believers are to be watchful in faith and expect His return at any hour.

Parables concerning the return of Christ: see Question 157.

“Then the kingdom of heaven shall be likened to ten virgins who took their lamps and went out to meet the bridegroom. Now five of them were wise, and five were foolish. [...] But while the bridegroom was delayed, they all slumbered and slept. And at midnight a cry was heard: ‘Behold, the bridegroom is coming; go out to meet him!’ Then all those virgins arose and trimmed their lamps. And the foolish said to the wise, ‘Give us some of your oil, for our lamps are going out.’ [...] And while they went to buy, the bridegroom came, and those who were ready went in with him to the wedding; and the door was shut. Afterward the other virgins came also [...] Watch therefore, for you know neither the day nor the hour in which the Son of Man is coming.”
Mathew 25: 1-13
cf. also Matthew 24: 43-51; 25: 14-30