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 theme of Obama's Inauguration is taken from a line in Lincoln's Gettysburg Address: "A New Birth of Freedom." "A New Birth of Freedom" commemorates the 200th anniversary of Abraham Lincoln's birth. The words, echoing across 200 years from Abraham Lincoln's Gettysburg address, express Lincoln's hope that the sacrifice of those who died to preserve the United States would lead to "a new birth of freedom" for the nation. Obama took his oath on the same Bible Abraham Lincoln used at his 1861 inauguration, when he proclaimed the end of slavery.