"Dydd Mercher dydd mawrth."
A comma would be usual, just as in English. And it should be a capital M in dydd Mawrth
Dangit, I put "Dydd Mercher, dydd Mauth"! "Mawrth" sounds like "Mouth"! How do I distinguish the difference?
The Welsh diphthong au is pronounced more like a British English 'eye' or 'aye'. The aw is more like the '-ow-'' in ''frown'. The correct sound is very well demonstrated in the Welsh pronunciation videos mentioned early on in the course notes and in the 'Popular' discussion thread titled 'Pronunciation/Ynganu'.
r is always pronounced as a distinct letter, unlike as in some English dislects.
It just takes constant listening practice! It is well worth going through those pronunciation videos at regular intervals and listening to as much Welsh as you can find.