The official flag of the United States of America was adopted June 14, 1777 by resolution of the Second Continental Congress. Believed to be celebrated first in 1861, in Hartford Connecticut, it was Woodrow Wilson, in 1916, who issued the proclamation that officially established June 14 as Flag Day. National Flag Day was established by an Act of Congress in August of 1949; however, Flag Day is not an official federal holiday. The week in which Flag Day falls is designated National Flag Week. Presidential proclamation is usually made to urge citizens to fly their national flags not only on Flag Day but the entire week.
Flags are symbols and have been popular through the ages. They generate feelings of intense national pride, patriotism, and respect for country. On the field of battle, the flag marks the warrior’s headquarters and is a rallying point. In 1861, a private in the 1st Tennessee Infantry Regiment recalled: ‘Flags made by the ladies were presented to companies, and to hear the young men tell of how they would protect the flag, and that they would come back with the flag or come not at all, and if they fell they would fall with their backs to the field and their feet to the foe, would fairly make your hair stand on end with intense patriotism, and we wanted to march right off and whip twenty Yankees.” (The Flags of the American Civil War 1: Confederate, by Katcher and Scollins, ISBN 9781855322707, page 3, para. 1).
The scientific study of the history and symbolism of flags is called vexillology. If you are interested, visit the North American Vexillological Association website where you can find information on flags from not only all parts of America but all over the world.