"V tamhletom bytě v pátek bylo padesát lidí."

Translation: There were fifty people in that apartment on Friday.

[tylerskarz]

Why isn't it "v pátku"?

[tylerskarz]

Because when indicating time or a day, 'v' takes the accusative. See Tips and Notes here https://www.duolingo.com/skill/cs/Numbers-2/tips-and-notes and 'v' on wiktionary https://en.wiktionary.org/wiki/v#Czech