Master Sgt. Cecilio Ricardo, U.S. Air Force/Wikimedia Commons

When Inauguration Day falls on a Sunday, the Constitution won't wait.

The iron beams are in place jutting out over Pennsylvania Avenue outside the White House.They will become the roof of the presidential reviewing stand for the second Obama inaugural next month.

But the traditional parade with its bands and floats will not march on January 20th, the day the Constitution requires the President to be sworn in. That falls on a Sunday this time so by tradition there will only be a small Sunday ceremony for the oath-taking at noon.

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This calendar clash happens every few decades.

For Ronald Reagan's second swearing in in 1985, a brief ceremony was held in the Grand Foyer on the state floor of the White House at 12 noon on Jan. 20, with a group of invited guests. It was televised live. But the parties and parades had to wait 24 hours.

President Obama will also have to take the oath twice: At noon Jan. 20 as the Constitution requires, and again Monday at the US Capitol with the whole world watching.

Watch: Swearing in the American President: Presidential Oaths 1961 -2005

He took the oath twice the first time as well, but that was because of a mistake. He and Chief Justice John Roberts stumbled over the wording of the oath, raising concerns that the swearing in might not meet the letter of the law. President Obama called Roberts to the White House Map Room the next evening and the oath was administered correctly in private, just to be safe.

Here's a look back at Obama taking the oath in 2009: