Is no longer useful or popular or successful, as in Some people think the railroad has had its day. This expression is the antonym of have its (or one's) day, meaning “to experience success or prosperity,” as in Every dog has its day, a proverb dating from the 16th century. Both terms use day in the sense of “a propitious or opportune time.”
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.