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.[1]

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,[4] 23 January in 2006,[5] 22 January in 2007,[6] 21 January in 2008,[7] 19 January in 2009[8] and 25 January in 2010.