The "tag då" is normally just attached to one-word phrases. Varför då? Vem då? När då? etc. In those cases, it's just there to fill out the phrase and doesn't change the meaning (or: it just makes it sound more normal), so we just skip it when translating into English.
If you attach då to a longer question, it changes the tone of the question. Often it becomes more impatient. This då can be translated into 'then' in English:
När då? = 'When?'
När kommer du då? 'When will you come then?' (or 'So when will you come?')