"When is your birthday?"
Translation:Wann hast du Geburtstag?
It's unidiomatic and sounds strange.
Direct translation between languages is often impossible- you can't translate the German directly to 'When do you have birthday' in English, either.
German and English are fairly different when it comes to birthdays!
I see this in my daughter, whose English is influenced by German, and one of the places it's especially noticeable is when talking about birthdays.
Emily hat morgen Geburtstag will become "Emily has her birthday tomorrow" rather than "It's Emily's birthday tomorrow", etc.
In English, you talk about when somebody's birthday is (the birthday being the subject) while in German, you talk about when somebody has (a) birthday (the birthday being the object and the person the subject).
Yet "Wann ist dein Geburtstag?" is actually accepted by Duolingo (even if it´s not idiomatically correct).
"Wann hast du dein Geburtstag?" was not accepted. Does it really sound wrong with "dein" included?
Of course; it's grammatically wrong :)
(Geburtstag is masculine and is the direct object of du hast; dein is masculine nominative or neuter accusative, so either wrong case or wrong gender.)
More seriously, though: even Wann hast du deinen Geburtstag? would sound unusual to me -- Wann hast du Geburtstag? is more common.
This discussion answers a question I had elsewhere about '...zum Geburtstag' v '....zu meinem Geburtstag'.
Like your daughter I guess enough practice may eventually get me to accept the (still unnaturally sounding) correct German phrase!