Memorial Day was declared a national holiday in the United States in 1971. It was declared that the day of remembrance be held on the last Monday in May so federal employees would have a three-day weekend. It honors all fallen military personnel who fought for our freedoms in all wars that the US was involved in.
It started out as Decoration Day and honored all soldiers who died in the Civil War. That changed after World War I.
You can read the history of the federal holiday here:
Memorial Day
