A more thorough explanation:

Nefastus is a Latin word that means "not lawful for public business." In ancient Roman law, it refers to a day when it is illegal to open the courts, administer justice, or hold public assemblies. The priests in charge of supervising the laws and religious observances established an official calendar, on which certain days, marked "nefasti," were to be devoted to religious or public ceremonies.

One example of a nefastus day is the Roman festival of Saturnalia, which was celebrated in December. During this festival, the courts were closed, and no public business could be conducted. Instead, people would exchange gifts, feast, and participate in other festivities.

Another example is the Lupercalia festival, which was held in February. On this day, the courts were closed, and people would participate in a purification ritual that involved sacrificing goats and dogs.

These examples illustrate how nefastus days were set aside for religious or public ceremonies and were not meant for conducting legal or governmental business.