星期几(xīngqī jǐ) - what day of the week? in Mandarin

星期几 (xīngqī jǐ) asks which day of the week it is, pairing 星期 (xīngqī), "week", with 几 (jǐ), "which number". It is the standard question for a day name, Monday through Sunday, rather than a calendar date.

The days themselves slot the same way a number would: 星期一 (xīngqī yī) is Monday, 星期二 (xīngqī èr) is Tuesday, and so on, so 今天星期几 (jīntiān xīngqī jǐ), "what day is it today?", is answered by simply replacing 几 (jǐ) with the right number, 今天星期三 (jīntiān xīngqī sān), "today is Wednesday".

Keep 星期几 (xīngqī jǐ) apart from 几月几日 (jǐ yuè jǐ rì), which asks for a calendar date rather than a weekday, and from 几号 (jǐ hào), the shorter, more colloquial way of asking for the day of the month.