"My acquaintance works every Monday."
Translation:Mój znajomy pracuje w każdy poniedziałek.
Not "always", but mostly, when it means "in". But there are differences depending on the real meaning. For example if something is literally in (inside) your head, it's Locative - "w głowie". If you hit yourself in the head, it's Accusative - "w głowę".
Anyway, while in this meaning it usually would be Locative - "works in a company" = "pracuje w firmie"; days of the week, and the word "weekend" are exceptions. They use Accusative. Which for the masculine ones will look the same as Nominative, as they are non-animate.