The Continental Congress adopted the Flag Resolution on June 14, 1777.
The resolution established that the United States flag would have thirteen alternating red and white stripes and thirteen white stars on a blue field. It did not specify the arrangement of the stars, which allowed later designs to vary.
The thirteen stripes and stars represented the original thirteen states. As new states entered the Union, Congress eventually increased the number of stars while keeping the number of stripes at thirteen. The modern flag therefore has fifty stars but still retains thirteen stripes.
June 14 is observed annually in the United States as Flag Day. The resolution does not prove that Betsy Ross designed the first flag; that popular story was promoted much later and lacks firm contemporary documentation. The resolution itself is the clear congressional source for the original national design.