Saint Patrick’s Dayn. March 17, observed by some Christians in honor of Saint Patrick, the patron saint of Ireland, and often celebrated secularly in honor of Irish culture and heritage.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.Saint` Pat?rick’s Day` n. March 17, observed in honor of St. Patrick, the patron saint of Ireland. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.