Hoy es un día muy especial para Sara. Su madre le compró un vestido elegante y todos sus amigos van a llegar a su fiesta de cumpleaños. Después de la fiesta ella ya no es una niña; ahora es una mujer. Hoy es la __________________.
Translation:
Today is a very special day for Sara. Her mother bought her an elegant dress and all her friends will arrive at her birthday party. After the party she is not a girl anymore; she is now a woman. Today is the [blank].
Two things to note here: 1. Sara's mother bought her an elegant dress, and 2. after the party Sara will be a woman. This describes two elements of the Latin American tradition of a quinceañera, which takes place on a girl's fifteenth birthday (hence quinceañera), and marks her transition from childhood into adulthood. It is the day of Sara's quinceañera.
Answer:
fiesta de quince años