Memorial Day is Monday, May 27 and honors those who have died while serving in the United States military.

Here are five things to know about the history of Memorial Day.

1. Memorial Day began in the years following the Civil War.

In the late 1860s, people who lost loved ones in the Civil War began gathering every spring to pay tribute to their friends and family who were killed during the war. It didn't become an official holiday until 1971.

2. Waterloo, New York is the official birthplace of Memorial Day.

Many other towns claim to be the birthplace of the holiday, but Waterloo is the only one officially recognized by the federal government as the birthplace. Waterloo first celebrated the holiday on May 5, 1866 and was declared the official birthplace of Memorial Day 100 years later. 

3. Memorial Day was originally known as Decoration Day.

In 1868, General John Logan called for a nationwide day of remembrance in May, writing that on "Decoration Day," Americans should decorate the graves of soldiers "whose bodies now lie in almost every city, village and hamlet churchyard in the land."

4. It hasn't always been observed on the last Monday in May.

Memorial Day was held every year on May 30 for decades, until Congress passed the Uniform Holiday Monday Act, decreeing that Memorial Day, a federal holiday, would take place the last Monday in May. So if you have Memorial Day off and a three-day weekend, you have Congress to thank for it.

5. The red poppy tradition stems from a World War I poem. 

"In Flanders Fields" by Lt. Col. John McCrea inspired the tradition of wearing a red poppy on Memorial Day. In 1915, the same year the poem was written, Georgia teacher and war volunteer Moina Michael wrote her own poem in response, calling for people to wear poppies to pay tribute to those who died in war.

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