"My acquaintance works every Monday."
Translation:Mój znajomy pracuje w każdy poniedziałek.
why isn't 'każdy poniedziałek' in the Locative case? don't we always use locative after 'w' ?
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.
"Mój znajomy" is masculine and "moja znajoma" is feminine, both should be accepted.
why isn't "każdego poniedziałku" accepted? In this thread it's explained that you can just use "każdej środy" so I don't see why it's not accepted here for a different day, or at the very least what I've done wrong