Saying "góðan dag" is just like a formal way to say hi, and it's usually said to someone who you are doing business with. For example if you call some company and someone answers "you say góðan dag to be polite". But hæ is more like informal you usually say to your friends and family. Like if you are coming into party, you always greet someone with "hæ", if you would say "góðan daginn" everyone will think you are weird. Me myself, I only use "Góðan daginn" (icelandic grammar, you usually say that to people) when I'm speaking to someone I've never spoke to before or/and I'm doing business with him/her, or if I'm greeting older co-workers that I don't know that well. Hope that helps