Here is a brief history behind Flag Day:
On June 14, 1777, the Continental Congress proposed that the United States have a national flag instead of the British Union Jack. The 13 stars of the flag represented the 13 new states. There were few public ceremonies honoring the Stars and Stripes until 1877, when on June 14, it was flown from every government building in honor of the centennial adoption of a national flag. Schools had unfurled American flags over their doors or outside the buildings long before this; but in 1890, North Dakota and New Jersey made a law that required their schools to fly the flag daily. The first official Flag Day was observed in Philadelphia, Pennsylvania in 1893. New York also proclaimed June 14 as Flag day in 1897. Other states were slow to follow. Some people thought the day was too close to Memorial Day and Independence Day.
In August 1949, President Harry S. Truman proclaimed June 14 as Flag Day. Since then the President proclaims the commemoration yearly, and encourages all Americans in the country to display the Stars and Stripes outside their homes and businesses!