A look at the inauguration, in question-and-answer form:
Q: Why is the inauguration on Jan. 21 instead of Jan. 20?
A: The Constitution's 20th Amendment, passed by Congress in March 1932 and ratified by the necessary states the following January, sets the inauguration date as Jan. 20 at noon.
Because that's a Sunday this year, Obama will take the official oath of office that day in a private ceremony. A public ceremony will be held Jan. 21 on the west front of the U.S. Capitol. Local officials are planning for an estimated 600,000 to 800,000 people to crowd onto the National Mall to witness the oath-taking ceremony.
This is the seventh time the inauguration date has fallen on a Sunday. Inaugural ceremonies, however, are not traditionally held on Sundays because courts and other public institutions are closed.
Before 1933, the president had been sworn in on March 4, typically the final day of the congressional season. But the stretch between the November elections and the March 4 inauguration led to a lengthy lame-duck sessions of Congress and became a concern during times of national crisis.
Sen. George Norris, R-Neb., suggested the 20th Amendment, which called for a new Congress to begin on Jan. 3 and for the president to be inaugurated on Jan. 20. President Franklin Roosevelt's first inauguration, in 1933, was the last swearing-in ceremony held on March 4.