"I swim on Thursday, Saturday and Sunday."
Translation:Ik zwem op donderdag, zaterdag en zondag.
When you use 's here so Ik zwem 's donderdags, 's zaterdags en 's zondags you are saying that you swim on Thursdays, Saturdays and Sundays and hence every Thursday, Saturday and Sunday, so reoccurring.
The current sentence however does not specify that it happens every Thursday, Saturday and Sunday and hence you use Ik zwem op donderdag, zaterdag en zondag which indicates it only happens once and is not a reoccurring event.
Firstly, if you want the plural form of Thursday, you write donderdagen, not donderdags. Same with Saturday and Sunday, it is -dagen. Next, the sentence is saying that they swim on those three days, not particularly every thurs, sat, and sun. If it were those same days every week, then it would be "op elke donderdag..." Or "op donderdagen..."
Your sentence is correct if the sentence was "I swim on Thursdays, Saturdays, and Sundays."
DonderdagS is used to describe something (A Thursday outing) or in your sentence's case, used to 'replace' op donderdagen. So the last sentence of my previous comment, it should actually say: If it were those same days every week, then it would be "op elke donderdag..." Or "op donderdagen..." Or "donderdags".
It really depends on the sentence. Most of the time we don't use the oxford comma, but sometimes we do. F.e. when the main sentence is too long we sometimes use it and also when there is an enumeration and we want to make differences between several words. For example Beauty and the Beast, and Peter Pan. Because Beauty and the Beast belong to each other, the comma makes it easier to understand that Beauty and the Beast belong to eachother.
Just to clarify, the Oxford comma only refers to listing three or more distinct things in a list and is not used to differentiate between different clauses. In English, you wouldn't normally put a comma between "Beast" and "and" in your example, because only two things are being listed; as you mentioned, "Beauty and the Beast" belong together (unless the individual characters are being named).