Learn Russian: Спокойной ночи, мама!

In Russian, when you wish someone something, like a good night, a happy journey, or good health, you usually use the genitive case. It acts as a short way of saying 'I wish you a...'.

What does спокойной literally mean?

It literally means calm or peaceful. Instead of using the word for 'good', Russians technically wish you a 'peaceful night'.

Ночь (night) is a feminine noun. Why does its genitive form end in -и (ночи) instead of replacing an -а with -ы?

Because ночь ends in a soft sign (-ь) rather than an -а or -я. For all feminine nouns that end in a soft sign, the genitive case ending is always -и.

Do I need to change the ending of мама because I am speaking directly to her?

No, you can just use the basic dictionary form (the nominative case) when addressing someone, which is why мама keeps its normal ending here. In very casual spoken Russian, you might hear people drop the last letter and shorten it to мам, but мама is always correct and natural.