星期三 means 'Wednesday,' literally 'week three,' as it is the third day of the week in the Chinese system where Monday is day one. It is one of the standard weekday terms used in everyday conversation and scheduling.
我们星期三下午有一个重要的会议。
Wǒmen xīngqīsān xiàwǔ yǒu yī gè zhòngyào de huìyì.
We have an important meeting on Wednesday afternoon.
星期三是我最忙的一天,课排得满满的。
Xīngqīsān shì wǒ zuì máng de yī tiān, kè pái de mǎn mǎn de.
Wednesday is my busiest day — my schedule is packed with classes.
你星期三有空吗?我们一起吃个饭吧。
Nǐ xīngqīsān yǒu kòng ma? Wǒmen yīqǐ chī gè fàn ba.
Are you free on Wednesday? Let's have a meal together.