On June 14, 1777, the Second Continental passed a resolution stating that “the flag of the United States be 13 stripes, alternate red and white,” and that “the union be 13 stars, white in a blue field, representing a new constellation.”
Over 100 years later, in 1916, President Woodrow Wilson marked the anniversary of that decree by establishing June 14 as Flag Day. Today we celebrate the anniversary of the adoption of the Stars and Stripes as the official US flag.
Original source can be found here.