The Collaborative International Dictionary of English v.0.48:

Inauguration Day \In*au`gu*ra"tion Day\
   The day on which the President of the United States is
   inaugurated, the 20th of January in every year next after a
   year divisible by four. Prior to the adoption of the
   twentieth amendment to the Constitution of the United States
   (ratified February 6, 1933) the date was the 4th of March.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

Inauguration Day
    n 1: the day designated for inauguration of the United States
         President [syn: Inauguration Day, January 20]