This is the 926th most frequent Japanese word.
週
Week.
In this sentence, '週' (week) is used to specify recurrence in weekly terms, '毎週' translates to 'every week'.
私は毎週月曜日に英語の授業を受けます。
I have English classes every Monday.
In this case, '週' is in '来週', meaning 'next week', referring to the week following the current one.
来週、学校で文化祭があります。
Next week, there is a cultural festival at school.
Here, '週' is used in '先週', meaning 'last week', to refer to the week preceding the current one.