Summary: The Second Sunday in June of each year is the beginning of National Flag Week. Here is a short Biblical perspective for Flag Day which is June 14th of each year.
Fly the Flag High
As a Christians' Banner
Psalm 20:5 says, "May we shout for joy over your salvation, and in the name of our God set up our banners! May the LORD fulfill all your petitions!" The word banner (דָּגַל dāg̱al) is a verb meaning to carry a flag, a standard, a banner: to distinguish. It indicates the display of a symbol of loyalty and commitment to the Lord for His victories. God's people are to show their loyalty to the Lord by raising their flags (banners) in His name. (Expositors Bible Commentary; Matthew Henry Commentary)
Countries design and display flags, not just to have something to fly in the air but to make a particular statement. Much time and money is invested in the design of a flag. Everyone who sees a flag displayed should know from that symbol just what the country stands for. Many Americans display flags outside their homes to show national loyalty, cultural pride, or affiliation with specific interest groups and organizations.
The American flag was designed as a banner to celebrate America's freedom, but also stands as an icon of world diplomacy. (Flags and Their Importance, United States Flag Store) The Word English Dictionary says that to fly the flag “is to represent or show support for one's country, an organization, etc."
On June 14th, 1777 the Continental Congress passed a resolution, "The flag of the United States will be thirteen stripes, alternating red and white; that the union be thirteen stars, white in a blue field, which will represent a new constellation." (Journals of the Continental Congress, Vol. VIII, p. 464, June 14, 1777) "Since that time, generations of Americans have celebrated the flag as a symbol of our God-given freedoms, as well as being a God-blessed nation. In every American military campaign, the flag has been a symbol of our freedom." (David Barton, The Real Story Behind Old Glory - 06-11-2012)
You might be interested in some U. S. flag facts, "The National Flag Day Foundation holds an annual observance for Flag Day on the second Sunday in June. The program includes a ceremonial raising of the flag, recitation of the Pledge of Allegiance with singing of the National Anthem. The week of June 14 is designated as 'National Flag Week.' During National Flag Week, the president will issue a proclamation urging U.S citizens to fly the American flag for the duration of the week. The flag should also be on all government buildings" (Wikipedia, Flag Day) The Pledge of Allegiance spoken during the ceremony says, "I pledge alliance to the flag of the United States of America and to the republic for which it stands, one Nation under God, indivisible, with liberty and justice for all." (Celebrating America's Freedom, U.S. Department of Veterans Affairs) The national anthem, The Star-Spangled Banner is sung at the annual ceremony as well. In the 4th stanza are these words, "Praise the Power that hath made and preserved us a nation! Then conquer we must, when our cause it is just, And this be our motto: "In God is our trust" And the star-spangled banner in triumph shall wave O'er the land of the free and the home of the brave." (National Anthem - United States Of America Anthem Text Lyrics)
Christians want the flag flown high because their study of the Bible and their respect for the Founding Fathers of America, enables them to appreciate our flag as a symbol of freedom brought forth by and sustained by God our Creator.