Making an excuse

Making an excuse is so difficult thing for me. I don't want to tell a lie. But sometime we have to tell a lie, because I hope people truth me. For example, my boss said that he might have to work late on Sartday night. Then he ask me working late on Sartday. No! No! No! It was crap I thought. We have been studying on weekend. So I need to tell him a white lie. I said that I planed to celebrate my mother's birthday on weekend, sorry about that. So my boss said that was ok.