‘To call it a day’ means to ‘stop whatever one is doing at least for the present’. ‘Let’s call it a day’ means ‘Let’s stop for the time being what we are doing’. It does not mean that you stop working for the day. It only means that you don’t want to continue with what you have been doing for some time. If you have been discussing a problem for some time and if you feel tired physically or mentally, you could say ‘Let’s call it a day’. This implies that the matter will be taken up and discussed again at an appropriate time.”