The inauguration of the president of the United States is a ceremonial event marking the commencement of a new four-year term of a president of the United States. The day a presidential inauguration occurs is known as "Inauguration Day" and occurs on January 20 (or 21st if the 20th is a Sunday). (Prior to the Twentieth Amendment, the date was March 4, the day of the year on which the Constitution of the United States first took effect in 1789. The most recent public presidential inauguration ceremony, the swearing in of President Frank Underwood to begin his first four-year term in office, took place on Friday, February 17th, 2017.
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.
When George Washington was inaugurated, the oath was administered by Robert Livingston, Chancellor of New York State, in 1789, and by William Cushing, Associate Justice of the Supreme Court, in 1793. Since Chief Justice Oliver Ellsworth swore in President John Adams, no chief justice has missed an Inauguration Day. When Inauguration Day has fallen on a Sunday, the chief justice has administered the oath to the president on the Sunday privately and then again the next day publicly.
When a new president takes over mid-term due to the death or resignation of a president, the oath of office is administered but formal, public inauguration events have not been held. The word "inaugurate" means to make a formal beginning of, and the fact that these presidencies (nine of them) were begun without an inauguration attests to the celebratory nature of the event, being passed on during these periods mourning the previous presidency.