That time of excellent Mars viewing coincides with opposition, when Mars is directly on the opposite side of Earth from the Sun. This lineup happens about every two years. During this opposition, Mars and Earth are closest to each other in their orbits.
Close Approach is when Mars and Earth come nearest to each other in their orbits around the sun. Close is a relative term. The minimum distance from Earth to Mars is about 33.9 million miles (54.6 million kilometers). However, that doesn't happen very often.
If Earth and Mars had perfectly circular orbits, their minimum distance would always be the same. However, they have elliptical (egg-shaped) paths.
In addition, gravitational tugging by planets constantly changes the shape of their orbits a little bit. Giant Jupiter especially influences the orbit of Mars.
Mars Close Approach happens about every 26 months. It is closely related to Mars Opposition and Mars Retrograde.
Since Mars and Earth are at their closest, it's generally the best time to go to Mars. Many Mars missions have taken advantage of the close distance to visit the red planet.
NASA's Mars 2020 mission, with its Perseverance rover, is currently en route to the Red Planet, with a landing scheduled for Feb. 18, 2021. Several other nations also have spacecraft headed to Mars.