Inauguration Day
In the United States the day that the President of the United States is sworn in and takes office. Since the ratification of Amendment XX in 1933 this takes place every four years on January 20th. Prior to this Inauguration Day was held every four years on March 4th.
In accordance with Article II, Section 1, Paragraph 8 of the Constitution, upon entering office, the President must repeat the following oath or affirmation: "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 oath is traditionally ended with, "So help me God," although for religious reasons some Presidents have said, "So help me."
The oath of office is traditionally administered by the Chief Justice of the United States