To CALL IT A DAY means to stop what you have been doing.
"I'm

getting a little tired now. Let's call it a day."
"After three marriages, he decided to call it a day and just stay single."
"I hate this job! I'll call it a day and quit."

To CALL IT A NIGHT means to stop what you have been doing in the evening or night, often in order to go to bed.
"It's getting late. I think I'll call it a night."
"Let's call it a night and continue in the morning."

We often use the phrase "let's call it a day (or night)" to let the other person know we would like to finish the conversation.