Because Welsh u is not pronounced like in English :)
In the south, I believe it's similar to the "i" in English "sit" when short, or like the "ea" in English "seat" when long. So it sounds exactly like Welsh "i".
In the north, the sound is a bit different - a bit further back in the mouth. But still not like "oo" in English "boot" -- that would be "w" in Welsh!
Vowels in Welsh can vary in length - this u happens to be a long one. There are rules about when vowels are long or short but they have many exceptions and variations, so it is probably easiest just to learn as you go along.
Dydd is never used with nos and can be left out with bore and prynhawn.
Thanks for pointing that out. We need to update the notes.