"Good night, Eleri Lingo."
Translation:Nos da, Eleri Lingo.
I typed "Nos dda, Eleri Lingo." and was corrected to "Nos da".
I had a look in the "Gweiadur" dictionary and it said that while nos is feminine, mae ‘nos dda’ wedi caledu yn ‘nos da’ drwy hir ddefnydd ac arfer which apparently means something like nos dda has hardened into nos da through long use and practice, i.e. convention is to use nos da without soft mutation.
In case anyone else is wondering :)
Another common example of this oddity is wythnos diwetha (last week).
It's always felt like a set phrase to me when said on its own w/o being part of a full sentence - similar to some konjunctive forms in English - and therefore not mutated. Which is strange as other set phrases of similar length without wider contexts are mutated.