星期日 (xīngqīrì) means 'Sunday' and is one of two common ways to say Sunday in Chinese, the other being 星期天. It is used in the same way as other days of the week and can appear in date expressions or scheduling contexts.
我们星期日去公园野餐吧。
wǒmen xīngqīrì qù gōngyuán yěcān ba.
Let's go to the park for a picnic on Sunday.
图书馆星期日下午不开门。
túshūguǎn xīngqīrì xiàwǔ bù kāimén.
The library is not open on Sunday afternoon.
他每个星期日都会去教堂。
tā měi gè xīngqīrì dōu huì qù jiàotáng.
He goes to church every Sunday.