the day on which the president of the United States is inaugurated, being January 20 of every year following a year whose number is divisible by four. Prior to the Twentieth Amendment to the Constitution (ratified February 6, 1933), it was March 4.
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British Dictionary definitions for inauguration day
the day on which the inauguration of a president of the US takes place, Jan 20
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012