Meaning/Usage: Used to express that the work day is over.

Explanation: "Call it" is often times used to make a decision. When you add "day" to this, then a person is making a decision to say that the day is over. This is mostly used to end a working day.

"Let's call it a day. I'm too tired to continue working."
"We can't continue working without Mike, so let's call it a day."
"It's already nine o'clock. Let's call it a day."

A: "How much more work do we have for tonight?"
B: "I think we finished everything for the day."
A: "Good. Let's call it a day then."

Other Common Sentences

"Let's turn in for the night."
"I think we should pick it up from the morning."
pick it up = continue