“Therefore be on the alert,
for you do not know which day your Lord is coming."
No one knows the day and hour of the Lord's coming. Jesus Himself said that even He did not know the time; that the Father will tell Him when the time has arrived (Mark 13:32). Because we don't know the time we must be on constant watch for His return. The Bible says we will meet Him in the air (1 Thessalonians 4:16-17), and that He will come with the saints (Zechariah 14:5). Bible scholars see this as "two" second comings: one in the air for the church, which we call the "rapture". And the other is coming to the "world" with the church -- what we call the Second Coming of Jesus.
"To watch for Christ's coming, is to maintain that temper of mind which we would be willing that our Lord should find us in. We know we have but a little time to live, we cannot know that we have a long time to live; much less do we know the time fixed for the judgment. Our Lord's coming will be happy to those that shall be found ready, but very dreadful to those that are not. If a man, professing to be the servant of Christ, be an unbeliever, covetous, ambitious, or a lover of pleasure, he will be cut off. Those who choose the world for their portion in this life, will have hell for their portion in the other life. May our Lord, when he cometh, pronounce us blessed, and present us to the Father, washed in his blood, purified by his Spirit, and fit to be partakers of the inheritance of the saints in light." (Matthew Henry)