"I listen to music on Friday."
Translation:W piątek słucham muzyki.
Does the preposition have to come first in the sentence? Can it be "Słucham muzyki w piątek"?
Yes, and it works. Although it sounds just a little bit off to me - the original sentence is as if it was an answer to "So... what are you doing on Friday?", and your version to "When do you listen to music?" and somehow it's strange that the answer is "on Friday" and not "on Fridays".
Why is piątek not in the locative case? I would've assumed it would be 'w piątku'.
Days of the week (and the word "weekend") take Accusative. Locative would seem more logical, but... it's just not how it's said.