The design of the flag has been modified 26 times officially since 1777. The 48-star flag was in effect for 47 years until the 49-star version became official on July 4, 1959. The 50-star flag was ordered by President Eisenhower on August 21, 1959 and was adopted in July 1960. It is the longest-used version of the U.S. flag and has been in use for over 54 years.

On June 14, 1777, the Second Continental Congress passed the Flag Resolution which stated: "Resolved, That the flag of the thirteen United States be thirteen stripes, alternate red and white; that the union be thirteen stars, white in a blue field, representing a new constellation." Flag Day is now observed on June 14 of each year. While scholars still argue about this, tradition holds that the new flag was first hoisted in June 1777 by the Continental Army at the Middlebrook encampment.

More Info: en.wikipedia.org