星期一 (xīngqīyī) means "Monday," the first day of the work week in Chinese. It follows the pattern 星期 (week) plus a number, where 一 represents the first day. This same pattern applies to all days of the week from Monday (星期一) through Saturday (星期六), with Sunday being 星期天 or 星期日.
星期一我有三节课,非常忙。
Xīngqī yī wǒ yǒu sān jié kè, fēicháng máng.
I have three classes on Monday; I'm very busy.
我们约好了星期一下午见面。
Wǒmen yuē hǎo le xīngqī yī xiàwǔ jiànmiàn.
We agreed to meet on Monday afternoon.
下个星期一是公司的年度会议。
Xià gè xīngqī yī shì gōngsī de niándù huìyì.
Next Monday is the company's annual meeting.