Welcome to English in a Minute, where we teach you all about idioms in American English.
Let's say you have worked all day, and you are really tried, like these people:
A: I am exhausted. I have been working at this since 7:00 a.m. this morning.
B: Me, too. I think it's time we call it a day.
A: I agree. Let's go home.
Call it a day!
When you "call it a day" in American English, it means you are ending work, play, or something you are doing that day. Often, it means you have finished your work, and you are going home.
You can use this phrase in any situation, and everybody in America will understand you.
And that's English in a Minute.