THIS LIVE EVENT HAS CONCLUDED
By Carrie Dann, Political Reporter, NBC News
President Barack Obama is set to be officially sworn in Sunday for his second term as president of the United States, ahead of Monday's public events.
In a small and short ceremony at the White House, he will recite the constitutionally mandated oath for the third of four expected times during his time in office.
When Obama first took the oath on Jan. 20 four years ago, he and Chief Justice John Roberts tripped up over the wording, raising concerns about whether the constitutional requirements were fulfilled to the letter of the law. Roberts went to the White House the next day and administered it again in full.
This time, Obama will recite it twice on purpose. Sunday's official swearing-in, conducted again by Roberts, will be the 57th inauguration of a president in American history.
The short Sunday ceremony will be held because the constitutionally mandated inauguration date of Jan. 20 falls on a Sunday, so Obama will take the oath a fourth and final time on Monday before hundreds of thousands of observers on the National Mall. The Monday ceremonies will include the president's inaugural address, a luncheon with the president and members of Congress and the traditional inaugural parade and balls.
Vice President Joe Biden was officially sworn in an about 8:20 a.m. ET Sunday.
The small weekend ceremony is a bit of a historical quirk, although today's swearing-in is the seventh in history to take place on a Sunday. The last instance occurred in 1985, when President Ronald Reagan was formally sworn in for his second term in office.
Beginning with the second inauguration of Franklin D. Roosevelt in 1937, new presidents and vice presidents have been sworn in on Jan. 20 due to the changes laid out in the 20th Amendment to the Constitution.
(Before that, inaugurations were typically held on March 4, as directed by the 12th Amendment. But controversy over the length of the lame duck period forced that ceremony to be moved up by law).
In addition, eight vice presidents have been administered the oath of office upon the death of a president. The hastily planned ceremonies have taken place in hotels, homes and -- famously, after the death of John F. Kennedy -- aboard Air Force One.
The oath of office for the president is set out in the Constitution.
Article II, Section 1, states as follows:"Before he enter on the execution of his office, he shall take 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."
Traditionally, two minor changes are made. First, the president taking the oath says his name after the first word ("I, Barack Hussein Obama"). And second, the phrase "so help me God" is added at the end.
The president will deliver his public inaugural address Monday on the East Front of the Capitol.
NBC's Pete Williams contributed to this report.