If you are looking for the translation of "night", the Welsh translation is "nos", which is assumed to be derived from the Latin "nox". It is a feminine noun with the most common plural form "nosweithiau" (you may also come across "nosau" as a plural)
In this way "nos da" would mean "good night", which you would say after leaving a party or a gathering, or when you had a chat with a neighbour or friends over the garden fence in the evening and you are retiring inside.
Another compound you may come across is "noson" as in "noson llawen" where it means "evening or night", in this case an evening get-together with singing and music, usually taking place in a pub or village hall etc.
"Noswaith" is usually only used as "noswaith dda" , a greeting, which means "good evening" and you use it when you join someone in the pub in the evening, or get some shopping from a late shop etc. The "waith" part of the word refers to "wythnos = week", which I always thought of as meaning "weekday evening" to help me remember it .
Hwyl