Just curious: my solution ("Nos dydd Sadwrn") was rejected, with a hint stating that "nos" replaces "dydd". This hasn't been introduced as such, as far as I know, and it breaks the pattern with "bore" and "prynhawn" which may, but don't always, stand in for the "dydd". However, I'm not sure whether this is a special rule, so any insight will be welcome.
It is always nos Sadwrn, nos Lun, nos Fawrth, etc, with no dydd.
As to why, perhaps it is because bore, prynhawn are parts of a dydd, whereas nos is not. In the end, though, it is just how the language has developed.