It’s back again: 21st January marks Blue Monday 2019. This infamous day gets a mixed reception when it rolls around, with some feeling concerned and others flouting it and treating it like any other day. After all, the whole idea of Blue Monday comes from a formula to define ‘the most depressing day of the year’ in order to boost sales. Even Cliff Arnal, the psychologist who devised the formula to pinpoint this gloomy day in the calendar, has vehemently…