It should say 'ddydd Llun' as there is a treiglad adferfol. See part 6.46 of 'Gramadeg y Gymraeg', Peter Wyn Thomas (t. 431-433).
Os ydach chi'n meddwl bod rhywbeth yn anghwyir dylech chi ei adrodd yn ôl i'r cyfranwyr trwy defnyddio'r botwm "report" oherwydd wedyn mae nhw'n gweld bod yna camgymeriad i'w gywiro.
that reply isn't a lot of help to me, a beginner! I thought the same thing, is it a colloquialism as we use in modern English?
Sorry I answered in Welsh because the original poster speaks fluently. In short I said I you think something is incorrect in the course you should ise the report button because then they can see where exactly in the course the error is and fix it. You should be able to use both "Dydd Llun" and "Ddydd Llun" here I believe.