Questions & Answers about Магазин закрывается в девять вечера.
In в девять вечера, вечера is the genitive singular of вечер. Time expressions with в follow the pattern:
• в + numeral in the accusative (here девять)
• + period‐of‐day in the genitive (here вечера)
So you literally have “at nine of the evening,” which in English is “at nine in the evening.”