星期四(xīng qī sì) - Thursday in Mandarin

星期四 (xīngqī sì) is Thursday: 星期 (xīngqī, "week") plus 四 (sì, "four"), the fourth day of the numbered run.

It works the same as every weekday, needing no preposition to mark time: 星期四我要交作业 (xīngqī sì wǒ yào jiāo zuòyè), "I have to hand in my homework on Thursday".

For a learner listening on the fly, 四 (sì) and 十 (shí, "ten") are the pair worth watching, since a mumbled or fast 星期四 (xīngqī sì) can be mistaken for something else entirely if the final consonant is not caught; slowing the speaker down or asking them to repeat is entirely normal here.