Today’s is the 56th presidential inauguration.
The 20th Amendment to the Constitution states that the “terms of the President and Vice President shall end at noon on the 20th day of January.” The Amendment was ratified in 1933 — the first inauguration on the new date was January 20, 1937.
Before the 20th Amendment, the Constitution did not provide the date when the terms began and ended. The terms of the first President and Vice President were fixed by an act of the Continental Congress adopted September 13, 1788. That act called for “the first Wednesday in March next to be the time for commencing proceedings under the Constitution.” It happened that the first Wednesday in March was the 4th day of March, and hence the terms of the President and Vice President and Members of Congress began on March 4, 1789. (Washington did not take the oath of office until April 30, 1789, but technically his term began March 4th.)
The Constitution set the terms of the President and Vice President at four years. Accordingly, any change from March 4th required a constitutional amendment because a date change would mean that the incumbents would not serve exactly four years. Franklin Roosevelt’s and John Nance Garner’s first terms were 43 days less than four years — March 4, 1933 – January 20, 1937.
Why did they change the date?
I think 20th century transportation and communication made the four months between election and inauguration seem unnecessary — and 1861 had proved it was dangerous. Movement for the amendment began well before the economic crisis of 1929-1932, though ratification wasn’t until January 1933.
The bigger change was for the congress to convene in January, rather than not until the following December (i.e., more than a year after the election).