"Call it a day" = To quit whatever you are doing, stop the activity for the day or for the present moment. Suppose I'm at work, I look at the clock and think "great ... it's 5 o'clock, let's call it a day !" By the way... wiiii !!! (I forgot to show my happiness =p)
Should we call it a day? ... in this case someone is asking someone else if he or she agree to finish what they are doing.