Peace Officers Memorial Day and Police Week is an observance in the United States that pays tribute to the local, state, and peace officers. The Memorial takes place on May 15, and Police Week is the calendar week in which the Memorial falls.
The holiday was created on October 1, 1961, when Congress asked the president to designate May 15 to honor peace officers. John F. Kennedy signed the bill into law on October 1, 1962. Amended in 1994, Bill Clinton, through Public Law 103-322, directed that the flag of the United States be flown at half-staff on May 15. According to a proclamation by George W. Bush in 2002,
"Peace Officers Memorial Day and Police Week pay tribute to the local, State, and Federal law enforcement officers who serve and protect us with courage and dedication. These observances also remind us of the ongoing need to be vigilant against all forms of crime, especially to acts of extreme violence and terrorism."
Much of the holiday centers on the National Law Enforcement Officers Memorial wall in Washington, D.C., whose walls feature the names of the more than 17,000 law enforcement officers who have been killed in the line of duty.
http://www.2011calendar.ca/images/calendar-may-2011.jpg (This is where the calendar came from)