President Barack Obama took the official oath of office Sunday in a semi-private ceremony in the White House.

Inauguration Day is always Jan. 20, but when it falls on a Sunday, public ceremonies traditionally occur the following Monday – and that’s the case this year.

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President Barack Obama issued a call to unity in his second inaugural address, urging the nation to move past the divisions that marked the last four years in politics and complete the work of living up to America’s founding principles.

The president, in a speech that blended together post-partisan rhetoric and policy declarations, highlighted the progress made during his first term to end foreign wars and turn around the economy.

But Obama said that there was much unfinished work ahead, and he used Monday’s speech to urge political leaders to finally rise above bitter squabbling — a recurring theme of his first term, and a mark of how difficult it has been for Obama to live up to his 2008 vow to change Washington’s business as usual.

Related: The full text of President Barack Obama’s inaugural address

“We cannot mistake absolutism for principle, or substitute spectacle for politics, or treat name-calling as reasoned debate,” Obama said. “We must act, knowing that our work will be imperfect.  We must act, knowing that today’s victories will be only partial, and that it will be up to those who stand here in four years, and forty years, and four hundred years hence to advance the timeless spirit once conferred to us in a spare Philadelphia hall.”

Obama and Vice President Joe Biden were sworn in by Chief Justice John Roberts and Supreme Court Associate Justice Sonia Sotomayor, respectively, shortly before noon; Monday’s oath of office was ceremonial, following their formal, constitutionally-prescribed swearing-in on Sunday.

MSNBC contributed to this report.