There's a novel (and short story) by Isaac Asimov called Nightfall. The premise is a world lit by multiple suns which only sees the stars every thousand years or so when each sun gets totally eclipsed at the same time. The people of the world experience true darkness for the first time in their memory, and they go mad. Only a secretive cult is prepared for nightfall, and they alone begin rebuilding the world time after time. It says a lot for the battle between science and "religion" and the virtue of preparedness.
We, on the other hand, get to experience night, well, nightly (unless you're in the polar regions). We're all used to the daily fading of the light and the emergence of the stars. There comes another, symbolic night, however. Jesus says this in John 9:4: "As long as it is day, we must do the work of Him who sent me. Night is coming, when no one can work." What sort of night is this? At first reading, the quick answer is eternity and the judgment is nightfall. No one can be saved after Christ returns. No one can preach or baptize. The work of the kingdom will be done, and no one can do anything else.
For this reason, we must work while it is still day. As long as time runs and the earth stands, there is work to be done. Now, while they can still be saved, we must tell others about the Savior. We have to show Christian love, walk in truth, and be salt and light. As the world grows darker, as it approaches nightfall, we must shine brighter. We don't know -- can't know -- when night will finally come. We don't know when the sun will set on this world, and a new world will be born which will have no need of a sun. And so we must work now, while it is yet day.
Do the work of an evangelist. Perform the service of a slave of Christ. Work, for night is coming, and our labors must end.