Mars oppositions happen couple of years but the eccentric orbit of Mars means some are better than others. Perihelic oppositions, like 2018, come along every 15 years or so and the opposition distance is close to minimum. However – distance isn’t everything!
This is my view of Mars from Northumberland during the 2018 opposition. Less than 10 degrees above the horizon at best. I’ve been out every clear night and the telescope view has been terrible! At low altitude Mars is distorted by the atmosphere. Even when the seeing is good – the light from Mars is refracted into a spectrum of colours so it looks more like a block than a disk! I can mitigate the effects of this to some extent using colour filters…but it isn’t great.
Things are much better in March 2020 at the next opposition: Mars is much higher in the sky but only marginally further away. Atmospheric distortion will be less – much better telescope views are virtually guaranteed! This is the *real* perihelic opposition for us northerners!