To call it a day is to decide you have worked enough and stop. Tools down, and go home.
It is used about a day's work and, more finally, about a career or a relationship, so a player calling it a day is retiring. That second use is one of the gentlest ways English has of announcing an ending. There is a variant, call it a night, used when the work or the party has run into the evening. The phrase takes no object and always stays in one piece.
1phrasecall it a dayTo stop working for the day, or to end something for good.
“It was getting dark so we decided to call it a day.”
From earlier call it half a day, which first appears c. 1834, meaning to leave before the working day was over.
Etymology from Wiktionary, CC BY-SA
Practised in Idioms, A Man Called Ove
First appears in chapter 5 of A Man Called Ove.
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