This will be the third time Obama has recited the constitutional oath since he took office in 2008, although Obama will repeat it again during the Martin Luther King Day ceremony.
Monday's public ceremony is expected to attract thousands of Americans to none-too-balmy Washington DC, where they will watch Obama deliver an address detailing his vision for his second official term.
Why is Obama being sworn in twice? It's a matter of both the calendar and constitutional law. According to the 20th Amendment to the US Constitution, Obama must begin his term at noon on January 20th.
Tradition is the reasoning behind the second event on Monday, after James Monroe decided back in 1821, after his Sunday swearing-in, that it would be better to hold the public event on a Monday.
The rationale? According to the US Government, it's because “courts and other public institutions were not open on Sunday.”
The Presidential Oath of Office may be important, but it's not very long: Obama must repeat "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."
Vice President Joe Biden was sworn on early Sunday morning at the Naval Observatory by Associate Justice Sonia Sotomayor, who he personally selected to deliver the oath — the fourth woman to do so in history.
Biden used the Biden Family Bible in the ceremony, which he's used since the first time he was sworn in as a Senator back in 1972. It's been in the family since 1893.