Sid is correct. I would just add that "call it a day" usually implies that you are leaving a task unfinished. This could mean that you are going to continue later (another day) or that you have run out of time and have to stop.

“Call it a day” means “let’s stop”, “I’ve had enough”, “let’s finish”. Usually associated with work, at some point during the working day someone decides it is time to stop, go home, today’s work has been done, let’s call it a day. Can be used in many other scenarios including studying, chores and even sport or leisure activities. "we played golf all day, then it started to snow, so we decided to call it a day"