"Friday, Saturday"

Translation: Dydd Gwener, Dydd Sadwrn

[RoKerslake]

Hello!
Here I was asked to translate "Friday, Saturday" into Welsh and I had read that you could call the days just by their names, but my answer ("Gwener, Sadwrn") was marked as incorrect.
Is there a rule for contexts in which it's not necessary to say "Dydd" before the name of the day?
Thanks! :)

[EllisVaughan]

The reason you need dydd before them is because the days of the week also apply to the eight planets (excluding earth which is "Y ddaear"). I personally would never say them without "Dydd" before them.

[RoKerslake]

Thanks a lot!
I was a bit confused because I read somewhere you could do without the word "Dydd."
The days of the week in Spanish also come from names of planets :)
Thanks again!