Glenda: I've been working on this report for 13 hours today.
Hank: You must be exhausted. When is the report due?
Glenda: In 3 days.
Hank: You have plenty of time to finish. Why don't you call it a day and start fresh tomorrow?
Glenda: That's a great idea.
Vinnie: Hey Tim, how's it going? Are we doing any business?
Tim: Not in a while. I haven't sold anything in 3 hours.
Vinnie: Oh, let's call it a day. Most people have gone home anyway.
Meaning: finish or be done. "To call it day" is most often used to indicate the speaker's work is finished (at least for that day). But when this expression is used, it also suggests work will (likely) continue the next day. Basically, it means the work day is done.