"His birthday" 可以说成 "His birthday" 或 "The day of his birth".
这个词组的来源是源自于英语生日的定义，就是一年中的某一天，一般是指一个人出生的那一天。
下面是一些例句：
"When is his birthday?"（他的生日是什么时候？）
"I'm throwing a birthday party for him on his birthday."（我正在为他举办一个生日派对。）
"He always celebrates his birthday with his family."（他总是和他的家人庆祝他的生日。）