David Rice Atchison: President for a Day
At the end of James Polk's term as President of the United States, his successor, Zachary Taylor was to have taken office. Unfortunately, the day Polk's term ended was a Sunday, and Taylor, for religious reasons, refused to be sworn in until the following Monday. So for Sunday, March 4, 1849, with Polk out of office and Taylor not yet in office, the Constitutional rules of succession required that Missouri senator and President Pro Tempore of the Senate David Rice Atchison take the awesome responsibilities of President of the United States for one day.
Behind the Legend
Senator David Rice Atchison was indeed considered, at least informally, to be President of the United States for March 4, 1849, for the reasons described in the legend. He used his executive powers in a playful manner for that day, calling for a national day of celebration of pandas, granting a raise in salary "to all gentlemen with attractive teenage daughters," declaring war on Lake Superior, and ending slavery. These executive orders were all rescinded the next day, but they provided a source of fun for a crabby nation.
Unfortunately, Atchison was not really President in a technical legal sense.
The Constitution of the United States of America, Article II, Section 1, reads (in part): "The executive power shall be vested in a President of the United States of America. ... Before he enter on the execution of his office, he shall take the following oath or affirmation: -- 'I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.'" The key here is that bit about the oath of office. No President, from the time of and including George Washington, has ever said, "swear (or affirm)" when giving the oath -- all have, in fact, left out the part in parenthesis. And because none of these men -- not even David Rice Atchison -- properly said the oath, none can be considered to have legally taken the Presidency of the United States, and the country has been without a President since its founding.