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 last inauguration to take place on the older date was Franklin D. Roosevelt's first one on March 4, 1933.) The most recent public presidential inauguration ceremony, the swearing in of President Barack Obama to begin his second four-year term in office, took place on Monday, January 21, 2013.
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.