Short answers, pulled from the story.
Roman public holidays fell into three categories: stativae (fixed-date festivals), conceptivae (moveable feasts announced by priests or magistrates), and imperativae (on-demand festivals called in response to unusual events or the need for expiation).
Ovid's Fasti is an incomplete poem that describes and provides origins for Roman festivals from January through June as they existed during the age of Augustus. It is one of the most important surviving sources for the study of Roman holidays.
During the feriae, public business was suspended, lawsuits and quarrels were forbidden, and slaves were to receive rest. The flamens and the Rex sacrorum were not permitted even to watch work being performed. Romans who inadvertently worked could pay a fine or offer a piaculum, usually a pig.
The Ludi Apollinares were first held in 212 BC as a single-day event on July 13 and established as an annual festival in 208 BC. They were held in honour of Apollo.
The English word "fair" derives directly from the Latin feria. The Romans held mercatus, or trade fairs, immediately after major ludi, and Cicero credited the semi-legendary king Numa Pompilius with establishing these markets alongside religious festivals.
Byron used the phrase to describe a dying gladiatorial figure being watched by his family, with the lines "Butchered to make a Roman holiday." By the early 19th century, particularly in response to the violence of the later French Revolution, the phrase had come to mean enjoyment derived from the suffering of others.