From Wikipedia on the most depressing day of the year:
Arnall says the date was calculated by using many factors, including: weather conditions, debt level (the difference between debt accumulated and our ability to pay), time since Christmas, time since failing our new year’s resolutions, low motivational levels and feeling of a need to take action. Writing about the calculation, Goldacre stated: … the fact is that Cliff Arnall’s equations … fail even to make mathematical sense on their own terms.
This date allegedly falls on the Monday of the last full week of January. The date was declared by Arnall to be 24 January in 2005, 23 January in 2006, 22 January in 2007, 21 January in 2008, 19 January in 2009 and 25 January in 2010.