"It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness..." -A Tale of Two Cities by Charles Dickens
"With malice toward none; with charity for all; with firmness in the right..." -Abraham Lincoln, Second Inaugural Address
It is a trope (the use of a word, phrase, or imagery in a way not intended by its normal signification)
Definition: a mild or indirect word or expression substituted for one considered to be too harsh or blunt when referring to something unpleasant or embarrassing
It can be used to mask a rude or impolite expression but conveys the concept clearly and politely
"Shall I compare Thee to a Summer's Day" -William Shakespeare (extended metaphor between the love of the speaker and the fairness of the summer season)
"The assignment was a breeze." (implies the assignment was easy)