Since the beginning of the year, Mars has had conjunctions with Jupiter (7 January), Saturn (2 April) and Pluto (26 April), but its closest approach to a planetary sibling this year occurs on Friday, 7 December 2018 at 14:07 UT (2:07pm GMT) when the Red Planet passes just 2 arcminutes – one-thirtieth of a degree – north of Neptune in the constellation of Aquarius.
The instant of least separation between Mars and Neptune occurs in daylight for the British Isles, but observers here only have to wait a further four hours to see them in a dark sky. As astronomical twilight fades to dark around 6pm GMT in the heart of the UK, Mars and Neptune are also at their highest in the sky due south, about 30 degrees above the horizon.By 6pm GMT on 7 December the separation of Mars and Neptune has increased to 6½ arcminutes, or about one-tenth of a degree. This means that you could theoretically use telescope magnifications up to about 400× and still see both planets in the same field of view! (In practice, seeing conditions at an altitude of 30 degrees rarely permits such powers; 150× would be more realistic.)
If you get the chance to see both Mars and Neptune in the same field of view on 7 December, do bear in mind that this conjunction is merely a line-of-sight effect. The magnitude +0.1 Red Planet lies almost 159 million kilometres away, hence its 86% illuminated gibbous disc measures just 8.8 arcseconds across. Neptune has a physical diameter 7¼ times that of Mars but lies slightly more than 28 times farther away this night, hence its pale blue magnitude +7.9 disc spans just 2.3 arcseconds.