Lucas 21:34-35

"That day" is the day of His return, not the destruction of Jerusalem, since it would come on all earth-dwellers (Luke 21:35). Jesus did not want His disciples to be unprepared for His return. He did not want them to be so self-indulgent and selfish that they disregarded His return. In that case it might catch them as a trap. Even though believers should be able to anticipate the Lord’s return by the signs that precede it (Luke 21:10-11; Luke 21:25-26), they may become so entangled in the affairs of life that they lose sight of it.