The story
After signs posted at town entrances warning that “colored people” (among other terms) were required to leave town by sundown.
Etymology from Wiktionary, CC BY-SA 4.0.
sundown town
Today it means: A place or jurisdiction that prohibits or discourages a certain class of people, typically a certain race or ethnicity, from living there or remaining there after sundown, or, in some cases, from visiting during the day. Full definition →