Yes - noswaith is not used for a 'named' night/evening.
On this course, I think you will only meet it in the greeting noswaith dda (good evening).
For a named evening/night it is always nos xxx (no dydd). Note the soft mutation following nos:
- nos Lun, nos Fawrth, nos Fercher, nos Iau, etc
See my earlier reply in this thread. It is also explained in the course notes for this section on 'Days'.
- nos Sadwrn = 'Saturday evening' or 'Saturday night'.