Blue Monday

Blue Monday is the name given to a day in January (typically the third Monday of the month) that is considered the most depressing day of the year. 

The term originated as a marketing campaign by UK travel companies to incentivize people to travel abroad.

${\displaystyle \frac{[W+(D-d)]\times T^Q}{M\times N_a}}$

where W = weather, D = debt, d = monthly salary, T = time since Christmas, Q = time since the failure of new year's resolutions, 

M = low motivational levels, and Na = the feeling of a need to take action. 

[From Wikipedia]