Example sentences
jīn今tiān天xīng星qī期jǐ几？
What day is it?
xià下xīng星qī期wǒ我qù去běi北jīng京。
I go to Beijing next week.
About this word
星 is star and 期 is period, so 星期 is literally a "star period". The name comes from an old seven-day cycle read off the sky: each day belonged to one of the seven moving lights, the sun, the moon, and the five planets visible to the naked eye. That is why the days count 星期一 through 星期六 and then close with 星期日 or 星期天 instead of a seventh number. For "week" as a unit of scheduling, 周 is the more common choice in writing: 下周, 每周.
The story behind the character
星
On top is a shortened 晶 (sparkle, for the stars), below is 生 giving the sound. A phonetic character for "star."
期
其 (sound) plus 月 (time). It meant an agreed time. Xu Shen wrote 期, 会也: "to meet." A time you've agreed to meet.