During last night's meetup the question came up as to why one says
nos da (good night) but
noswaith dda (good evening). I thought it was because
noswaith was feminine and
nos masculine, but recourse to my dictionary disposed of that hypothesis - both are feminine. I promised to research the matter, and this is what I found:
nos is indeed feminine, and logically should be followed by
dda, but (according to my grammar books) it is one of a small number of feminine Welsh nouns ending in "s" which don't cause soft mutation in the following adjective.
Wythnos (week) is another example. As a side note, it appears that "night" nouns in general tend to be feminine - e.g.
nos (night),
noson (evening, night),
noswaith (evening, night),
noswyl (eve of a religious feast). (References: Stephen J. Williams,
A Welsh Grammar; Peter Wynn Thomas,
Grammadeg y Gymraeg;
Geiriadur Prifysgol Cymru).
-GRG