xīngqīyī
Monday
Chinese numbers its weekdays: 星期 ("week") plus a digit. Monday is day one, Saturday is day six, and Sunday has a name of its own.
Grammar rule: Days of the week with 星期
jīntiān xīngqī jǐ?
What day is it today?
jīntiān xīngqīwǔ.
Today is Friday.
xià xīngqīyī
Next Monday
shàng ge xīngqī
Last week
Monday is 星期一 (xīngqīyī), literally "week one". The short form is 周一 (zhōuyī).
Sunday is 星期天 (xīngqītiān) in speech and 星期日 (xīngqīrì) in writing.
Both mean "week". 星期 is the everyday word; 周 is shorter and more formal: 周一, 周末.
Tomyo
Numbers stick when you use them. Tomyo reviews your words on a schedule so what you learn is still there a month later.