Resolved questions
Why is the genitive case used in "good night", but not in "good morning"..., etc.?

Hi everyone! First I put up this post in English instead of other languages, in order to attract the most audience and language experts around. Anyone may feel free to answer me in the languages (given below) that I understand if he/she feels more comfortable: français, español, Deutsch, italiano, português, 日本語(にほんご), 한국말, and(和) 中文(ㄓㄨㄥ‧ㄨㄣˊ)。[Sorry, please, not in Russian, because I am only a beginner :(.]

I started learning Russian through some evening courses. Sometimes, the teacher may not be able to take care of so many questions within a lesson. Hence, I have a question regarding this language.

Here are some greeting expressions in Russian:
1) good morning = до́брое у́тро (in nominative case);
2) good afternoon = до́брый де́нь (in nominative case);
3) good evening = до́брый ве́чер (in nominative case);
4) good night = споко́йной но́чи (in genitive case).

How I would like to ask, why is the genitive case used in ONLY the expression "good night", while NOT in the others? I understand that "споко́йной" comes from "споко́йный", which means "quiet, clam, tranquil", and "до́брый" (and "до́брое" is the declension on the neutral nouns) means "good". Both are adjectives in nature. However, I do not understand, why one set (до́брый) takes on the nominative case, while the other set (споко́йный) takes on the genitive case.

In case that such similar question has been posed and answered, please kindly accept my sincere apology, and wherever possible, please kindly refer me to the corresponding weblink.

Any constructive answers and comments are warmly welcomed.

Thank you very much for your kind attention, and a good week to all.

Additional Details:

I should clarify that for points 1) to 3) in my original post ("до́брое у́тро", "до́брый де́нь", and "до́брый ве́чер"), they may be also in the accusative case, since for non-animated nouns, the Russian regulations state that their nominative and accusative cases have the same format. I am sorry should I have stated it wrong.

Additional Details:

Thank you very much for all the given answers. After knowing the reasons, I now feel more comfortable regarding the corresponding cases used.