The 20th Amendment to the U.S. Constitution- changing the commencement
of the terms of the President, Vice President from 4 March to 20 January and the commencement of terms of Members of Congress (both Senators and Congressmen) from 4 March to 3 January as well as the date for convening regular sessions of Congress from the first Monday in December to 3 January- was
declared ratified on 6 February 1933, before the 73rd Congress had even
taken office the following 4 March and it was, thus, already known- when
that Congress began- that the new date of 3 January for convening sessions of Congress and the start of terms of its Members would be the date of convening the 2nd session of the 73rd Congress.
Accordingly, newly inaugurated President Franklin Delano Roosevelt
invoked his authority to call Congress into "extra" session (under
the pre-20th Amendment schedule, the 1st regular "long" session of the 73rd Congress would not otherwise have begun until 4 December 1933)- this 1st session becoming known as the famous "Hundred Days" of the early New Deal era- but it was, from its beginning, treated as being, in fact, the
newly-established "odd" session under the soon-to-be-effective 20th
Amendment (hence its being listed herein as both an "extra" session and a regular "odd" session).