NASA planned for Viking 1 to land on Mars on July 4, 1976, during the United States Bicentennial.
The Viking program sent two orbiters and landers to Mars to study the planet's atmosphere, surface, geology, and possible biological activity. Viking 1 reached Mars in 1976, but its landing was delayed until July 20 because engineers needed additional time to assess the landing site and spacecraft conditions.
July 20 was itself a highly symbolic alternative date: it was the seventh anniversary of Apollo 11's first crewed lunar landing. Viking 1 successfully landed in Chryse Planitia on that day, while Viking 2 later landed at Utopia Planitia.
The planned July 4 landing is a notable example of the way NASA tied scientific milestones to the Bicentennial calendar. It is also easy to confuse Viking 1 with Viking 2, but Viking 1 was the mission originally targeted for the Independence Day landing.