星期一(xīng qī yī) - Monday in Mandarin

星期一 (xīngqī yī) is Monday, built the same simple way every day of the Mandarin week is built: 星期 (xīngqī, "week") plus a number. 星期一 (xīngqī yī) is literally "week one".

The pattern runs straight through the week: 星期二 (xīngqī èr, "Tuesday"), 星期三 (xīngqī sān, "Wednesday"), 星期四 (xīngqī sì, "Thursday"), 星期五 (xīngqī wǔ, "Friday"), 星期六 (xīngqī liù, "Saturday"), and then Sunday breaks the numbering, taking 星期日 (xīngqī rì) or 星期天 (xīngqī tiān) instead of a "seven".

星期一 (xīngqī yī) needs no preposition to work as a time word: 星期一我要去学校 (xīngqī yī wǒ yào qù xuéxiào), "I have to go to school on Monday", simply sits before the verb, the way "on Monday" sits at the front of the English sentence but without an equivalent for "on".