a day, usually the third Sunday in June, set aside in honor of fathers.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for father's day
a day observed as a day in honour of fathers; in Britain the third Sunday in June
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
Word Origin and History for father's day
1910, begun in Spokane, Washington, U.S., but not widespread until 1943; an imitation of Mother's Day.
Online Etymology Dictionary, © 2010 Douglas Harper