The closest distance from the earth to Mars is about 55 million kilometers, and the farthest is more than 400 million kilometers. Only when the angle between the earth and Mars is 70° can the probe reach Mars as scheduled. 
my country is expected to launch a Mars probe in 2020, and the following year. Landing on Mars, an aerospace expert commented: If this opportunity in 2020 cannot be seized, then the next suitable launch time will be delayed until at least 2022.
The relative near point of Earth and Mars occurs about every 15 years, and the year when the two are close is the best time to land on Mars