Please correct my incorrect or incomplete understanding: I was under the impression that dydd (Sul) was softly mutated (ddydd Sul) when referring to something ON a day. Does that only refer to an action, and not conditions such as the temperature?
In spoken Welsh you don't really hear the difference between 'ddydd Sul' and 'dydd Sul', so we've not used the mutated form in the early part of the course, although it is accepted as an answer.
In a way it's analogous to the abbreviated expression in spoken English leaving out the 'on'.
The mutation dydd Sul - ddydd Sul is because it is being used as an adverb 'on Sunday' qualifying the verb. Not everybody does this in colloquial Welsh, but it is regarded as Cymraeg da ('good Welsh') to do so.