This is the 6576th most frequent Chinese word.
星期日
Sunday
In this sentence, '星期日' is used as the subject to refer to the day Sunday.
星期日是我最喜欢的一天。
Sunday is my favorite day.
Here, '星期日' is applied as a temporal noun to specify the time of an action.
我通常在星期日和家人一起吃饭。
I usually have meals with my family on Sunday.
In this case, '星期日' serves as a possessive noun to describe something happening on Sunday.