Althoughnos is a feminine noun, the expected soft mutation ofda is absent because of an old rule (now inoperative) that disallowed/ð/ after another dental consonant. Compare alsowythnosdiwethaff(“last week”) with the same fossilized non-mutation.[1]
Nos da is used upon retiring to bed or of an evening when one will not see another until the next day. In the latter situation, it may be used slightly earlier in the day than is usual for the corresponding English phrase, especially in the early darkness of a late winter afternoon.
