Today a new President and Vice President will be sworn in, but why today?  The first inauguration, that of George Washington, took place on April 30, 1789.  All regular inaugurations from 1793 until 1933, were held on March 4, the day of the year on which the federal government began operations under the U.S. Constitution in 1789. The exception to this pattern was those years in which March 4 fell on a Sunday.  Inauguration Day moved to January 20, beginning in 1937, following ratification of the 25th amendment, where it has remained since. 

It has been held publicly on this date, January 20th, with the exception of three years, when the 20th fell on a Sunday.  In those years the oath was administered in a private ceremony and the public swearing in happened on Monday the 21st.  The only part of the inauguration that is mandated by the constitution is the reciting of the Presidential oath of office, typically administered by the Chief Justice of the Supreme Court.  The rest of the day’s events have been added over the years.