周 + Number
The noun 周四 (zhōusì) means 'Thursday' and is formed by combining 周 (week) with 四 (four), since Thursday is the fourth day of the Chinese week starting from Monday. An alternative form is 星期四 (xīngqīsì), which is equally common in everyday speech.
我们周四下午开会,别忘了。
Wǒmen zhōusì xiàwǔ kāihuì, bié wàng le.
We have a meeting Thursday afternoon — don't forget.
周四我有空,一起吃饭吧。
Zhōusì wǒ yǒu kòng, yīqǐ chīfàn ba.
I'm free on Thursday — let's have dinner together.
这个快递周四就能到。
Zhège kuàidì zhōusì jiù néng dào.
This package will arrive by Thursday.