President Barack Obama will take the oath of office Sunday surrounded by family in a private inauguration at the White House.
The ceremony is required by the Constitution, which says presidents automatically begin their new terms at noon on Jan. 20.
But, because it falls on a Sunday, organizers scheduled a second public swearing-in for Monday.
A crowd of up to 800,000 people is expected to gather on the National Mall to witness that event.
Supreme Court Chief Justice John Roberts will swear the president in both days.
Vice President Joe Biden was sworn in Sunday morning in a small ceremony at his residence, the Naval Observatory.