Why Sunday? Obama's dual inauguration ceremonies honor tradition and law

Washington (CNN) - President Barack Obama joins a rare collection of presidents on Monday. No, not the fraternity of 21 second-term presidents, but the even more exclusive group of seven presidents whose inaugurations have fallen on a Sunday.

By holding a private ceremony–before live television cameras but without a public audience–just before noon on Sunday and a full ceremony on Monday, Obama is following with both the tradition established by his predecessors and the legal obligations the Constitution outlines for inaugurating U.S. leaders.

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Ronald Reagan was the last to do the same, for his second inauguration in 1985. January 20th that year fell on a Sunday as well and Reagan was sworn-in for a second term privately in the North Entrance Hall of the White House. The public ceremony was to be held on the West Front of the Capitol on Monday but freezing weather forced most of the outdoor events to be cancelled and Reagan's inaugural address was moved inside to the Capitol Rotunda.

The Twentieth Amendment to the Constitution, ratified in 1933, laid out the rules of inaugurating a president, including the fact that "the terms of the President and Vice President shall end at noon on the 20th day of January" and "the terms of their successors shall then begin."

Before President Franklin D. Roosevelt's 1937 inauguration–the first one held in January–most presidents were inaugurated on March 4, the date set by an act of Continental Congress in 1788 and an act of Congress on March 1, 1792, according to The United States Capitol Historical Society.

Realizing that four months was quite a long time to hand over power between administrations, the Twentieth Amendment looked to speed up that transition period. By mandating the actual day on which the president must be inaugurated, the chance of that day falling on a Sunday became real. So after 1933, the inauguration couldn't be moved just a day later.

That is the constitutional reasoning for why Obama will technically be sworn in on Sunday, January 20, in the Blue Room of the White House.

The reason for Monday's pomp and circumstance? Tradition.

President James Monroe's second inaugural in 1821 fell on a Sunday, the first time that had happened for the young nation. After consulting with the Supreme Court on whether he could be inaugurated on Sunday, Monroe decided to hold the ceremony on Monday because "courts and other public institutions were not open on Sunday," according a release from the Presidential Inaugural Committee.

At the time, there was no constitutional guideline on when a president has to be inaugurated, so moving the swearing in a day was not violating America's founding document.

Presidents Zachary Taylor, in 1849, followed Monroe's lead and held the ceremony exclusively on Monday.

Rutherford B. Hayes broke from tradition by taking the oath of office privately on Saturday, March 3 in the White House Red Room and then again publically on March 5, 1877. With Hayes' Saturday wearing-in, he became the first president to take the oath of office in the White House.

President Woodrow Wilson also broke from tradition when, in 1917, he took the private oath on Sunday and had a public ceremony and speech on Monday. This was before the Twentieth Amendment, however, so Wilson had the option to hold all events, like his predecessors, on Monday.

After Wilson, two former presidents – Dwight D. Eisenhower in 1957 and Reagan in 1985 – followed his lead and held a private swearing in on Sunday and public events on Monday.

On Monday, Obama will become the third.