I am exhausted, let's ______ a day?

Call at

Call it

Call in

Call of

I am exhausted, let's call it a day.

میں تھک گیا ہوں چلو آج کا کام یہیں ختم کرتے ہیںThe correct phrase is "call it a day," meaning to stop working or end an activity.

It typically because you're tired or have accomplished enough for the day.