* Informal: "I'm exhausted. Let's call it a day and go home."
* Formal: "We've accomplished our goals for the day. It's time to call it a day and return to our offices."
* Humorous: "After three hours of trying to assemble this bookshelf, I'm calling it a day. Maybe I'll just hire a professional."
* With a hint of regret: "I was really looking forward to finishing the project, but I'm calling it a day. I just need to get some rest."
* As a suggestion: "We've been at it for hours. Why don't we call it a day and pick it up tomorrow?"