They are interchangeable for nos Sul/Lun/Fawrth/etc.
See the course notes for the sections on 'Days'. dydd is not used with nos - it is one or the other.
To see how to find the course notes generally, go to https://forum.duolingo.com/topic/924/hot and read the discussion 'Course tips and notes'. The 'duome' link there is useful for browsing all the notes in one place. We recommend reading the notes for each new section as you start it.