On June 14, 1777, the Second Continental Congress took a break from writing the Articles of Confederation and 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 officially establishing June 14 as Flag Day.

Source: History