This is the 2139th most frequent Spanish word.
cumpleaños
Birthday.
In this sentence, 'cumpleaños' is used as a noun to indicate the occasion of someone completing another year of life.
Hoy celebramos el cumpleaños de mi hermana.
Today we celebrate my sister's birthday.
Here 'cumpleaños' is used to refer specifically to the event of someone's birthday.
Tengo un regalo para ti por tu cumpleaños.
I have a gift for you for your birthday.
The usage of 'cumpleaños' here denotes the specific day on which someone completes another year of life.