A more thorough explanation:

festum

A festum is a special day that is celebrated as a holiday or festival.

• Christmas is a festum that is celebrated on December 25th every year.
• The Fourth of July is a festum that celebrates the independence of the United States.

These examples show how festum is used to describe special days that are celebrated as holidays or festivals. Christmas and the Fourth of July are both examples of festums because they are days that are set aside for celebration and are recognized as important holidays.