"His birthday" 用英语表达为 "his birthday".
其来源是英语中"birth"一词，意为"出生"；"day"一词，意为"天"。因此，"birthday"就是指一个人的生日。
一些例句：
What's the date of his birthday? 他的生日是什么日子？
Let's celebrate his birthday next week. 让我们下周庆祝他的生日。
I bought him a gift for his birthday. 我为他的生日买了一份礼物。