History

Timeline
Very much like the history of America, since the late 1700's the flag has undergone many changes and seen many events for which it stands. The world has come to understand that it is a symbol of freedom and power. I have compiled the dates and events I have found from various sources to demonstrate the changes or events of the the flag.
Date Event
1776 : January 1 - The Grand Union flag is displayed on Prospect Hill. It has 13 alternate red and white stripes and the British Union Jack in the upper left-hand corner (the canton).
1776 : May - Betsy Ross reports that she sewed the first American flag.
1777 : June 14 - Continental Congress passed an Act to establish an official flag for the new nation: Resolved: that the flag of the United States be thirteen stripes, alternate red and white; that the union be thirteen stars, white in a blue field, representing a new constellation. (stars represent Delaware, Pennsylvania, New Jersey, Georgia, Connecticut, Massachusetts, Maryland, South Carolina, New Hampshire, Virginia, New York, North Carolina, and Rhode Island)
1787 : Captain Robert Gray carries the flag around the world on his sailing vessel (around the tip of South America, to China, and beyond). He discovered the Columbia river and named it after his boat The Columbia. His discovery was the basis of America's claim to the Oregon Territory.
1795 : Flag with 15 stars and 15 stripes (Vermont, Kentucky)
1814 : September 14 - Francis Scott Key writes "The Star-Spangled Banner." 
1818 : Flag with 20 stars and 13 stripes (it remains at 13 hereafter) (Tennessee, Ohio, Louisiana, Indiana, Mississippi) Act of April 4, 1818 - provided for 13 stripes and one star for each state, to be added to the flag on the 4th of July following the admission of each new state.
1819 : Flag with 21 stars (Illinois)
1820 : Flag with 23 stars (Alabama, Maine) first flag on Pikes Peak
1822 : Flag with 24 stars (Missouri)
1836 : Flag with 25 stars (Arkansas)
1837 : Flag with 26 stars (Michigan)
1845 : Flag with 27 stars (Florida)
1846 : Flag with 28 stars (Texas)
1847 : Flag with 29 stars (Iowa)
1848 : Flag with 30 stars (Wisconsin)
1851 : Flag with 31 stars (California)
1858 : Flag with 32 stars (Minnesota)
1859 : Flag with 33 stars (Oregon)
1861 : Flag with 34 stars; (Kansas) first Confederate Flag (Stars and Bars) adopted in Montgomery, Alabama
1863 : Flag with 35 stars (West Virginia)
1865 : Flag with 36 stars (Nevada)
1867 : Flag with 37 stars (Nebraska)
1869 : First flag on a postage stamp
1877 : Flag with 38 stars (Colorado)
1890 : Flag with 43 stars (North Dakota, South Dakota, Montana, Washington, Idaho)
1891 : Flag with 44 stars (Wyoming)
1892 : "Pledge of Allegiance" first published in a magazine called "The Youth's Companion." Authorship was claimed for James B. Upham and Francis Bellamy. In 1939 the United States Flag Association ruled that Bellamy was the author of the original pledge. 
1896 : Flag with 45 stars (Utah)
1908 : Flag with 46 stars (Oklahoma)
1909 : Robert Peary places the flag his wife sewed atop the North Pole. He left pieces of another flag along the way.
1912 : Flag with 48 stars (New Mexico, Arizona) Executive Order of President Taft dated June 24, 1912 - established proportions of the flag and provided for arrangement of the stars in six horizontal rows of eight each, a single point of each star to be upward.
1931 : Congress officially recognizes `The Star-Spangled Banner' as the national anthem of the United States . Its stirring words were written by Francis Scott Key.
1945 : The flag that flew over Pearl Harbor on December 7, 1941, is flown over the White House on August 14, when the Japanese accepted surrender terms.
1949 : August 3 - President Truman signs bill requesting the President call for Flag Day (June 14) observance each year by proclamation.
1954 : June 14 - The words, "under God" were added to the Pledge of Allegiance.
1959 : Flag with 49 stars (Alaska) Executive Order of President Eisenhower dated January 3, 1959 - provided for the arrangement of the stars in seven rows of seven stars each, staggered horizontally and vertically. Executive Order of President Eisenhower dated August 21, 1959 - provided for the arrangement of the stars in nine rows of stars staggered horizon tally and eleven rows of stars staggered vertically.
1960 : Flag with 50 stars (Hawaii)
1963 : Flag placed on top of Mount Everest by Barry Bishop.
1969 : July 20 - The American flag is placed on the moon by Neil Armstrong.
1995 : December 12 - The Flag Desecration Constitutional Amendment is narrowly defeated in the Senate. The Amendment to the Constitution would make burning the flag a punishable crime.
Current : The National Museum of American History has undertaken a long-term conservation project of the enormous 1814 garrison flag that survived the 25-hour shelling of Ft. McHenry in Baltimore by British troops. This flag inspired Francis Scott Key to compose "The Star-Spangled Banner." The flag had become soiled and weakened over time and was removed from the Museum in December 1998. Conservation efforts began in June 1999, starting with the removal of the linen support backing that was sewn into place in 1914 using 1.7 million stitches. Painstaking steps must be taken to preserve the flag, including pH readings to measure the levels of acid or base in the fabric, color readings to analyze dyes in the fabric and fiber analysis through microscopic examination. A thorough vacuuming of all surfaces and large-format photographing of every section of the flag to benchmark its condition must also occur before conservation measures are undertaken.

Resources on History
Below are a list of web sites that contain more details regarding the history of the American flag.