Jesus taught them how to know when his return was near.

Luke 21:29-33

29 Then Jesus told his disciples this parable: “Think about the fig tree, and all the other trees. 30 As soon as you see their leaves beginning to sprout, you know that summer is near. 31 Similarly, when you see these things that I have just described happening, you will know that it is almost time for God to truly rule as king. 32 Keep this in mind: All the things that I have just now described will happen before all the people who have observed the things that I have done have died. 33 You can be certain that these things that I have told you about will happen. That they will happen is more certain than that the earth and sky will continue to exist.”