National Flag Day commemorates the adoption of the United States flag on June 14, 1777.
Betsy Ross has been given credit for creating the first American flag. Since 1777 the flag has been modified 26 times. The last time was on August 21, 1959; ordered by President Eisenhower.

On May 30, 1916, President Woodrow Wilson issued a proclamation deeming June 14 as Flag Day.
President Wilson selected this day because “It is the anniversary of the day upon which the flag of the United States was adopted by the Congress as the emblem of the Union” and he wrote, “On that day rededicate ourselves to the nation, ‘one and inseparable’ from which every thought that is not worthy of our fathers’ first vows in independence, liberty, and right shall be excluded and in which we shall stand with united hearts.”

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http://www.nationaldaycalendar.com/days-2/national-flag-day-june-14/