Night and day are antonyms, meaning they are opposite in meaning. However, there are several synonyms for the phrase "night and day" that can be used to express the same concept of something being drastically different or contrasting. Such synonyms include "polar opposites," "day and night," "black and white," "light years apart," "worlds apart," "chalk and cheese," and "apples and oranges." Each of these phrases conveys the idea of two things that are vastly different from one another and serve to underscore the stark contrast between them.