This is the 487th most frequent German word.
geburtstag
"Geburtstag" means "birthday."
The word 'Geburtstag' is used as a noun referring to someone's birthday, in this case, the speaker's birthday.
Heute feiern wir meinen Geburtstag.
Today we are celebrating my birthday.
Here, 'Geburtstag' indicates the occurrence of someone's birthday, mentioning the day it falls on.
Ich habe am Sonntag Geburtstag.
I have my birthday on Sunday.
The usage of 'Geburtstag' here represents the specific date or time related to someone's birth day and is used within a question to inquire about it.