Definition of Memorial Day noun from the Oxford Advanced Learner's Dictionary

 

Memorial Day

 noun
noun
BrE
 
; NAmE
 
 
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a holiday in the US, usually the last Monday in May, in honour of members of the armed forces who have died in war see also Remembrance Sunday, Veterans Day Culture Memorial Day began in the South during the Civil War when flowers were placed on graves, and the North first celebrated it in 1868. It was formerly known as Decoration Day. Memorial Day traditionally begins the summer holiday from school. Southern states also celebrate Confederate Memorial Day on different dates during the year, especially 26 April, 10 May or 3 June.
See the Oxford Advanced American Dictionary entry: Memorial Day