Viking 1 was originally scheduled to land on Mars on July 4, 1976, during the United States Bicentennial.
NASA selected the date because it would have created a dramatic connection between the national anniversary and the first successful American landing attempt on Mars. After Viking 1 entered Mars orbit on June 19, its cameras photographed the planned landing region.
Those images showed that the primary site was too rough for a safe landing. NASA therefore delayed the touchdown while engineers and scientists searched for a safer location. Viking 1 eventually landed on July 20, 1976, at Chryse Planitia, the same date as the seventh anniversary of Apollo 11’s first Moon landing.
Viking 1 became the first spacecraft to achieve a successful landing on Mars and return surface images and scientific data. Viking 2 followed with its own landing in September 1976, but it was Viking 1 that carried the original Bicentennial landing plan.