This is the 2041st most frequent Korean word.
생일이
"생일이" means "birthday is" in English.
The word '생일이' is used to indicate possession with '-이' as the subject marker for the word '생일' (birthday), denoting that the birthday belongs to the speaker.
제 생일이 오늘이에요.
My birthday is today.
Here, '생일이' refers to 'a birthday' and '이' identifies it as the subject of the sentence.
친구의 생일이 언제인가요?
When is your friend's birthday?
In this context, '생일이' again functions as the subject to specify the father's birthday.