You may wish to refer to a system map as you read this:
On 7/4/99, Columbia Heights and Georgia Avenue stations were not yet open (nor was anything south of Anacostia). The Green Line ran in 2 sections: Anacostia to U Street and Fort Totten to Greenbelt. As a result, Metro was able to use the Mount Vernon pocket to turn one line (Yellow) and U Street (both platform tracks) to turn 2 lines (Blue and Green). This meant that both Huntington and Franconia-Springfield had direct service to downtown. A shuttle was set up to serve Arlington Cemetery. The outer green line was routed onto the Red Line, the practice during rush hours, to provide direct service downtown.
Columbia Heights and Georgia Avenue opened on September 18, 1999. As a result, U Street could not be used as a terminal on July 4th without causing major backups. Blue Line trains ran to Mount Vernon Square and Yellow Line trains ran to Rosslyn like the 1999 shuttle, since Huntingon and Eisenhower have far fewer passengers than Franconia-Springfield and Van Dorn, making it more practical to have the Huntington branch riders change trains. The Green Line to Farragut North did not run either since it was obsolete, both on July 4th and during rush hours.
The pattern has remained the same since then except the Blue and Yellow have swapped designations so that instead of keeping their branch names in VA (Blue to F/S, Yellow to Huntingon), they retain their downtown line names (Blue to Rosslyn and if it were normal service, to Metro Center, Yellow to Gallery Place). In addition, starting July 4th, 2001, the Green Line ran to Branch Avenue instead of Anacostia since the line to Branch Avenue was complete.
The purpose of this "revolutionary" pattern on July 4th is to make travel more efficient for getting people to the festivities downtown and home. It was decided the regular daily routings were not nearly as good as these are.