Right answer: The feast day of the saint for whom someone is named
Onomástico (Onomastics in english) is an adjective relating to names given to people at birth according to an specific catholic saint. This word refers to the celebration of the Saint in honor of the person's name.
In other words, it is the day in which, according to the Catholic saints, it is the festival of the saint in honor of whom the name was given to someone.
It should be noted that this word is not the same as birthday, since birthday is the anniversary of the day a person was born and onomastic is the celebration of the saint in honor of whom the name was given to that person, and not always the day of the saint matches the anniversary date.