To declare that a given day's work has been accomplished and go home; to quit for the day/night. To stop working for the rest of the day/night.
"Let's call it a day," the boss said, "and go out for a drink."
Herb tried to repair his car engine all morning before he called it a day and went fishing.
It was nearly midnight, so Mrs. Byron decided to call it a day, and left the party, and went home.
The four golfers played nine holes and then called it a day.
We've been working hard on this project all evening; let's call it a night.