The swearing-in of the President of the United States occurs upon the commencement of a new term of a President of the United States. The United States Constitution mandates that the President make the following oath or affirmation before he or she can "enter on the Execution" of the office of the presidency:

    I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.

The newly elected or re-elected President traditionally adds "so help me God" to the constitutionally mandated statement.

The swearing-in traditionally takes place at 12 noon local time (UTC-5 hours) on Inauguration Day at the United States Capitol in Washington, D.C., with the Chief Justice of the United States administering the oath. From the presidency of Martin Van Buren through Jimmy Carter, the ceremony took place on the Capitol's East Portico. Since the 1981 inauguration of Ronald Reagan, the ceremony has been held at the Capitol's West Front. The inauguration of William Howard Taft in 1909 and Reagan in 1985 were moved indoors at the Capitol due to cold weather. Until 1937, Inauguration Day was March 4. Since then, Inauguration Day has occurred on January 20 (the 1933 ratification of the Twentieth Amendment changed the start date of the term).

Since Chief Justice Oliver Ellsworth swore in President John Adams, no Chief Justice has missed a regularly-scheduled Inauguration Day swearing-in. When Inauguration Day has fallen on a Sunday, the Chief Justice has administered the oath to the President either on inauguration day itself or on the preceding Saturday privately and the following Monday publicly. Eight presidential deaths and Richard Nixon's resignation have forced the oath of office to be administered on other days and (with the exception of Gerald R. Ford's swearing in following Richard Nixon's resignation) by other officials. The War of 1812 and World War II forced two swearings-in to be held at other locations in Washington, D.C.

From 1789 through 2005, the swearing-in has been administered by 14 Chief Justices, one Associate Justice, three federal judges, two New York state judges, and one notary public. Though anyone legally authorized to administer an oath may swear in a President, to date the only person to do so who was not a judge was John C. Coolidge, Calvin Coolidge's father, a notary whose home the then-Vice President was visiting in 1923 when he learned of the death of President Warren G. Harding.

For more, visit:
Wikipedia's United States Presidential Inauguration page