Was when Viking 1 was *supposed* to land on Mars, but the mission team decided
the original landing site was too rough.

Of course it would have been a very nice capper to the United States celebrating
the Bicentennial of our Declaration of Independence on July 4, 1976, but too much
had been put into th Viking landers to risk such a thing. Besides, the historic landing
might also have found itself relegated to a less important status in the public view
with everything going on that day.

So they waited and ended up landing Viking 1 on another historic day, this one
even more appropriate - July 20, the anniversary of the first manned landing
on the Moon with Apollo 11.

For the serious details, see here:

http://www.hq.nasa.gov/office/pao/History/SP-4212/on-mars.html