The inauguration of the President of the United States occurs upon the commencement of a new term of a President of the United States.

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.

(... from Wikipedia on 2010-09-03 12:51:11 )