Today, festivals and holidays are often a time to let loose and suspend social norms. It is obviously not ok to dress like you would at Coachella in the real world. This was also the case in ancient Rome.
Saturnalia was a festival that took place in late December to celebrate the God Saturn. There was a sacrifice at the Temple of Saturn, a public banquet and lots of partying.
Certain social customs were also reversed. For example, masters provided table service for their slaves. And slaves were able to be disrespectful to their masters and not get in trouble for it.
Gambling was also permitted during this time. Saturnalia is likely the best known Roman holiday and was celebrated into the 4th and 5th century. Considering they fall around the same time of year, it could also have influenced Christmas.