What term is used for a child’s naming day?

The term "Dies Lustricus" refers specifically to a Roman ceremony that occurs on the eighth day after a child's birth. This event is significant because it is predominantly recognized as the child's naming day, where the child is officially given a name and welcomed into the family and society. During the ceremony, various rituals were performed, including offerings and possibly a presentation at the family’s household shrine. This tradition highlighted the social importance of naming and the transition of the child into the wider community.

In contrast, the other terms do not pertain directly to the naming ceremony. "Primordia" could be interpreted to reference the beginnings of life or creation, "Dies Novus" translates to "New Day" but does not specifically relate to naming, and "Familiae" refers to family but does not denote a specific ritual or event associated with a child’s naming. Therefore, "Dies Lustricus" is the most accurate term for a child's naming day within Roman cultural practices.