In the United States, Flag Day is celebrated on June 14. It commemorates the adoption of the flag of the United States, which happened that day by resolution of the Second Continental Congress in 1777.
In 1916, President Woodrow Wilson issued a proclamation that officially established June 14 as Flag Day; in August 1949, National Flag Day was established by an Act of Congress.
Flag Day is not an official federal holiday, though on June 14, 1937, Pennsylvania became the first (and only) U.S. state to celebrate Flag Day as a state holiday, beginning in the town of Rennerdale. Title 36 of the United States Code, Subtitle I, Part A, CHAPTER 1, § 110 is the official statute on Flag Day; however, it is at the President's discretion to proclaim officially the observance.
For more, see http://en.wikipedia.org/wiki/Flag_Day_(United_States)