"Are you going to Aberteifi Monday?"

Translation: Dych chi'n mynd i Aberteifi dydd Llun?

[crepuscularr]

Why is it "Wyt ti'n mynd i Aberteifi dydd Llun?" but "Dych chi'n mynd i Aberteifi ddydd Llun?" ?

[EllisVaughan]

Both the unmutated and mutate "dydd" should be accepted with both of them.

[GilesNunn]

Why should a mutation be considered correct here?

[mizinamo]

Two explanations I have seen are:

• it is from "ar ddydd Llun" (on Monday), with the mutation being due to the preposition "ar" (on), which later dropped out but left the mutation behind
• it is a grammatical feature to turn "dydd Llun" (Monday) into an adverb "ddydd Llun" (on Monday)

That said, some dialects do not use the soft mutation here even in the "on Monday" sense.