Today is a year before US presidential inauguration day. We’re very sure of this.

A year from today, the US will inaugurate a new president. But inauguration day has not always been thus fixed.

In the early years of the Republic, habit (rather than statute) placed the date of inauguration at March 4—though even that convention was not quite firm. In 1821, with the incumbent President James Monroe about to take the oath of office for his second term, March 4 fell on a Sunday. Monroe asked of Chief Justice John Marshall whether he could take the oath on the following day, rather than sully the Lord’s day with secular business.

Marshall replied,

I have conversed with my brethren.… As the constitution only provides that the President shall take the oath it prescribes ‘before he enter on the execution of his office,’ and as the law is silent on the subject, the time seems to be in some measure at the discretion of that high officer. There is an obvious propriety in taking the oath as soon as it can conveniently be taken, & thereby shortening the interval in which the executive power is suspended. But some interval is inevitable.

Let me interject: note that Marshall here says that, on consultation with his fellow Supreme Court justices, it appears inevitable that there be a period during which there is not a legitimate President of the United States. (I know what you’re thinking: what if ISIS had attacked in the wee hours of March 4, 1821?)

The time of the actual President will expire, and that of the President elect commence, at twelve in the night of the 3d of March. It has been usual to take the oath at mid day on the 4th. Thus there has been uniformly & voluntarily an interval of twelve hours during which the Executive power could not be exercised.… Undoubtedly, on any pressing emergency the President might take the oath in the first hour of the 4th of March, and the public business would sustain no injury by its being deferred till the 5th, no impropriety is perceived in deferring it till the 5th.

Congress took a more punctilious, if wilier, approach to the expiry of its term; conceding that its ability to legislate ended at midnight on March 3, the lawmakers were in the habit of setting the official clock back to show that midnight had not yet arrived. Later, recognizing the habit of inaugurating the president at noon on March 4, Congress adopted the notion that it was itself entitled to act up until that time.

The twentieth amendment of the US Constitution put an end to these lax practices. Passed by Congress on March 2, 1932 and at last ratified by the states on January 23, 1933, it fixed the meeting of a new Congress at noon on the third of January, and the end of the presidential term at noon on January 20.¹ Which is what it will be in 2017.

¹Now, inasmuch as this amendment was not ratified until January 23, 1933, it did not apply to the term of the then-outgoing President, Herbert Hoover, whose term was the last to extend until noon on March 4, which meant that Franklin Roosevelt’s first term was cut uniquely short, lasting from March 4, 1933, to January 20, 1937.

Moreover, the twentieth amendment also declared, “If, at the time fixed for the beginning of the term of the President, the President elect shall have died, the Vice President elect shall become President.” Giuseppe Zangara shot at President-elect Roosevelt on February 15, 1933.² If he had hit Roosevelt and killed him, because the twentieth amendment became part of the Constitution a few weeks previously, Vice President-elect John Nance Garner of Texas would have been inaugurated on March 4, 1933, in Roosevelt’s stead.

²A long-standing tradition, not well supported by documentation, has it that Zangara never meant to kill Roosevelt at all, but was employed by an organized criminal syndicate to murder Chicago mayor Anton Cermak, who was standing next to Roosevelt, and whom Zangara did lethally hit that day.

Much of what appears here comes from

Charles Warren, “Political Practice and the Constitution,” University of Pennsylvania Law Review and American Law Register 89, no. 8 (June 1941): 1003-1025.