also: きげんのよい
expression, i-adjective (yoi/ii class)
Describes a person's temporary cheerful state. Often used as a prenominal modifier (e.g., 機嫌のいい人).
今日は機嫌のいい日だ。
Today is a day when I'm in a good mood.
機嫌のいい彼はよく冗談を言う。
When he's in a good mood, he often tells jokes.
機嫌がいい is a full clause meaning 'is in a good mood', while 機嫌のいい is the attributive form used before nouns.
Phrase composed of 機嫌 (mood) + の (attributive particle) + いい/よい (good).