This is one of the easiest things to learn in Chinese. Chinese use the word 'week'(星期-xīngqī) and the numbers(1-6) for days of the week:
星期一(xīngqī yī) - Monday
星期二 (xīng qī èr)-Tuesday
星期三(xīng qī sān)-Wednesday
星期四(xīng qī sì)-Thursday
星期五(xīng qī wǔ)-Friday
星期六(xīng qī lìu)-Saturday
For 'Sunday' there are two choices: 星期天(xīng qī tiān) or 星期日(xīng qī rì)