Sundown towns... were all-white municipalities or neighborhoods in the United States... The term came into use because of signs that directed "colored people" to leave town by sundown.

The towns of Minden and Gardnerville in Nevada had an ordinance from 1917 to 1974 that required Native Americans to leave the towns by 6:30 p.m. each day. A whistle, later a siren, was sounded at 6 p.m. daily, alerting Native Americans to leave by sundown. In 2021, the state of Nevada passed a law prohibiting the appropriation of Native American imagery by the mascots of schools, and the sounding of sirens that were once associated with sundown ordinances. Despite this law, Minden continued to play its siren for two more years, claiming that it was a nightly tribute to first responders.

https://en.m.wikipedia.org/wiki/Sundown_town