You can find out more about that here.
Personally I don't do maths (well only in a pathetic finger-counting rubbish sort of way), so equations are out for me. But really, is mid - late January each year really so much more depressing for people than any other time of year? And are the factors quoted in the equation likely to affect most of the population in this same way at this same time each year?
At The Guardian Dean Burnett calls it a "depressing day of nonsense science (again)".
On a more serious note, for those people that experience SAD (Seasonal Affective Disorder) then any day with low light at this time of year could potentially be challenging. There is more information about SAD here. Mental Health Research UK is running a campaign called Blooming Monday to encourage people to dress more brightly to raise awareness about SAD. (There is a bright blue streak in my otherwise purple scarf if that counts....)
But back to Blue Monday. All kinds of organisations have taken advantage of the media hype to promote their own ways of overcoming the blues of this particular January Monday. If a man in blue lycra takes your fancy, then there's even a video: