Translation:She is going to the office on Wednesday.
Shouldn't dydd be mutated? On Wednesday = ddydd Mercher.
'On Wednesday' does mean 'ddydd Mercher' but I think because in this instance the translation is asking for just 'Wednesday', the answer is just 'dydd Mercher'. The mutation comes from the 'on'.
You could just as well write ’on Wednesday’, the role of the day in the sentence is still the same.