The inauguration of the president of the United States takes place on January 20, 2019. 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; Montgomery and Prince George's Counties in Maryland; Arlington and Fairfax Counties in Virginia, and the cities of Alexandria and Fairfax in Virginia, and who are regularly scheduled to perform non-overtime work on Inauguration Day. There is no in-lieu-of holiday for employees and students who are not regularly scheduled to work or attend school on Inauguration Day. 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. (With material from: Wikipedia)