Jesus told his disciples to be ready because they do not know the time of his return.

Matthew 24:36-44

36 “But neither I, nor any other person, nor any angel in heaven, knows either the day or the hour when the things that I have told you about will happen. Only God, my Father, knows. 37-39 37-39It will be like what happened when Noah lived. Until the flood came, the people did not know that anything bad would happen to them. Before the flood waters covered the earth, the people were eating and drinking as usual. Some men were marrying women and some parents were giving their daughters to men to marry them. They were doing all this until the day that Noah and his family entered the big boat. And then the flood came and drowned all those who were not in the boat. Similarly, the unbelieving people will not know when I, the one who came from heaven, will return, and they will not be expecting me. 40 When I return, I will not take all people up to heaven. I will take only those who trust in me. For example, two people will be in the fields. One of them will be taken {I will take one of them} up to heaven and the other person will be left {and I will leave the other person} here to be punished. 41 Similarly, two women will be at the mill grinding grain. One of them will be taken up to heaven and the other will be left. 42 So, because you do not know what day I, your Lord, will return to the earth, you need to be ready for me to return at any time. 43 You know that if the owners of a house knew at what time in the night thieves would come, they would be awake and prevent the thieves from breaking into their houses. Similarly, I will come just like unexpectedly as a thief. 44 So you need to be ready for my return, because I, the one who came from heaven, will return to the earth at a time when you do not expect me to come.”