“I’ve been at work since early this morning, I’m going to call it a day and go home.”
When you decide to finish the workday. Alternatively, when you decide to finish a sociable evening:
“We’ve been out since 9pm – at the restaurant, then the bar, now back at my friend’s. It’s 3am. I’m tired and drunk. I’m going to call it a night and go home. Bye everybody!”
The medial malleolus 