* Viking 1 entered Mars orbit on June 19, 1976, and its lander touched down on July 20, 1976.
* Viking 2 entered Mars orbit on August 7, 1976, and its lander touched down on September 3, 1976.
It's important to distinguish between the orbiters and the landers. The orbiters were spacecraft that circled Mars and took pictures, while the landers were the parts of the mission that actually touched down on the Martian surface.
