Nightfall is the time in the evening when it begins to get dark and night comes on. It is not midnight, and it is not exactly sunset: sunset is when the sun goes down; nightfall is a little later, when the light has really gone. The word sounds a bit like a story or a news report — “before nightfall” — more than like chat between friends, who might just say “before it gets dark” or “at dusk.” Farmers, hikers and writers like it because it names a clear deadline: finish the work, reach the village, get home, while you can still see.