Saturn, as seen by the Cassini spacecraft from orbit around it. Image courtesy of NASA.
The superior planets of the Solar System – Mars, Jupiter, Saturn , Uranus and Neptune – are those that orbit further out from the Sun than the Earth, as distinct from the inferior planets, which orbit closer to the Sun.
Unlike the inferior planets, the superior planets cannot pass in between the Earth and the Sun – that is, they never undergo an inferior conjunction – but they can still pass behind the Sun – undergoing a superior conjunction – and do so roughly once a year.
At the midpoints between these conjunctions, they appear directly opposite the Sun in the sky, and are said to be at opposition.
At these times, they rise at nightfall and set at sunrise, and are visible for much of the night, making them ideally placed for observation. At around the same time, they also make their closest approaches to the Earth, so that they appear at their largest and brightest.
In reality, most of the superior planets orbit so much further out in the Solar System than the Earth that their distances from us barely change over the course of the year as the Earth orbits around the Sun.
For example, Saturn's distance from us only varies between 8.5 and 10.5 AU, and so the primary significance of the date of Saturn's opposition is that that is the date on which it is visible for the greatest fraction of the night.
However, the same is not true of Mars, the closest superior planet to the Earth. Mars' orbit lies not far outside that of the Earth: its radius is around 1.5 times that of the Earth's orbit. At opposition, Mars can lie at a distance of as little as 0.5 AU from the Earth, but at conjunction, it lies at a distance of 2.5 AU. Mars presents a very much larger disk at opposition than it does at any other time, and must be observed within a few weeks of opposition for best results.