昨天可以说是快乐的一天。

Yesterday can be said to be a happy day.

youdao

每天都是快乐的一天。

All the days are happy days.

youdao

那是我一生中最快乐的一段时光。

That was one of the happiest moments of my life.

《牛津词典》

Being happy, in other words, is also a moral state in the sense of actually contributing to other people's wellbeing.

感受快乐，换言之，也是一种道德状态,对他人的幸福作出贡献。

哈佛公开课 - 幸福课课程节选

He is using the English translation of the German word frohlich from Nietzsche's The Gay Science.

他用的其实是德语“快乐的“一词的英文翻译,而那个词则来自尼采写的“快乐的科学家“这个标题。

耶鲁公开课 - 文学理论导论课程节选

He called the North a, quote, "Happy state of mediocrity, a hardy race of free, independent republicans."

他说北方是,"快乐的平庸之地,这里住着一群自由独立的共和主义者"

耶鲁公开课 - 美国内战与重建课程节选