"Dydd Llun dw i'n mynd i Norwy."

Translation: On Monday I am going to Norway.

[Lundgren8]

Shouldn’t this be ddydd Llun?

[ibisc]

Dw i'n mynd i Norwy ddydd Llun would definitely be correct, although not always found in the colloquial language. When the adverb is emphasised by being at the front of the sentence it often used not to be mutated, but it is apparently becoming more common to do so in some cases.

[Lundgren8]

Ah, thank you!

[Windrammer]

Croeso :)