"Their birthday" in Engpsh is usually written as "their birthday". The phrase comes from the noun "birthday", which refers to the anniversary of the date on which a person was born. Here are some example sentences:

Today is John's birthday.
I'm throwing a surprise party for my friend's birthday.
Let's sing "Happy Birthday" to the birthday girl/boy.

这句话的来源是名词 "birthday"，它指的是一个人出生的纪念日。以下是一些例句：

今天是约翰的生日。
我要为我的朋友举办一个生日惊喜晚会。
让我们为生日女孩/生日男孩唱 "生日快乐"。