"I like cooking on Friday."
Translation:Ik kook graag op vrijdag.
I understand what you mean, but what is written is Friday/vrijdag. If someone meant that it's a continual thing or that it's more than that one Friday, then it should be, "I like cooking on Fridays." That should be the proper sentence. However, "I like cooking on Friday" is still grammatically correct and doesn't sound weird because the difference is just one letter!
When I first read the sentence, I too thought the implication you mentioned, but then I noticed it was singular, so I thought the sentence was a little wonky in real life sense, but knowing how Duolingo is, I knew they only wanted vrijdag, implication or not.