Спокойный means calm, добрый means kind. In this group of idioms, night (ночь) is usually used with спокойный in genitive case (it's a contraction of "I wish you good night" - Я желаю тебе спокойной ночи), while other times of the day are used with добрый in nominative case (it's a statement "This is a good morning" - Это "доброе" утро)

Yes, per other replies on other pages:

ann666
You have three genders in Russian - masculin (Добрый), feminine (до́брая) and neuter (до́брое). And word " утро" is neuter.

MenyaZovutMegan
Yes, it is. утро is neuter, день and вечер are both masculine. The feminine version of добрый is добрая.

"Доброе утро, папа." is "Dzień dobry, tato." in Polish. So, I don't get it? Or do you mean the form: did you think it was going to be "добрый"? Day is masculine in Polish and Good day is used for Good morning there. Morning or "утро" is neuter in Russian. Day is masculine in Russian also. https://en.wiktionary.org/wiki/%D1%83%D1%82%D1%80%D0%BE https://en.wiktionary.org/wiki/dzie%C5%84

Did you put "Добрый день"? https://en.wiktionary.org/wiki/%D0%B4%D0%B5%D0%BD%D1%8C

http://forvo.com/search/%D0%B4%D0%BE%CC%81%D0%B1%D1%80%D0%BE%D0%B5/ http://forvo.com/search/%D0%B4%D0%BE%CC%81%D0%B1%D1%80%D1%8B%D0%B9/