difference between 'fødselsdag' and 'bursdag'
both meaning of fødselsdag and bursdag is the same. But when is fødselsdag used and which situation is bursdag used in? fødselsdag seems to be used when you fill out formal form like application documents. For bursdag, I think it is used in informal or private situation like when you throw a party. Is that right? Can you explain it more in detail?

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Answer

You're right at that. I think Fødselsdag is most used in formal situations because it's more understandeble. 
And bursdag was originaly gebursdag, this is an older word, more used by the elder in Norway and in dialects. Also, I've seen people use gebursdag instead of bursdag in more formal settings.
But i guess Fødselsdag also is more used because it sounds better and cleaner, and is more corect "bokmål".