expression
two days a week
Fixed expression used adverbially or as a noun phrase to indicate a weekly frequency of two days.
彼は週二日ジムに通っている。
He goes to the gym two days a week.
この授業は週二日あります。
This class meets two days a week.
Means 'three days a week', a higher frequency than 週二日.
Compound of 週 (week) and 二日 (two days), forming a fixed adverbial expression.