The inauguration of the president of the United States takes place on January 20th. The only inauguration element mandated by the United States Constitution is that the president make an oath or affirmation before that person can “enter on the Execution” of the office of the presidency. However, over the years, various traditions have arisen that have expanded the inauguration from a simple oath-taking ceremony to a day-long event, including parades, speeches, and balls.
Inauguration Day is a Federal holiday observed only by federal employees who work in the District of Columbia; and certain counties in Maryland and Virginia. The primary reason for the holiday is to relieve traffic congestion that occurs during this major event.
In addition to the public, the attendees at the ceremony generally include members of Congress, Supreme Court justices, high-ranking military officers, former presidents, living Medal of Honor recipients, and other dignitaries. (from: Wikipedia and cute-calendar.com)