Monday, January 24th, according to Cardiff University scholar Cliff Arnalls, who came up with an equation to calculate when the most depressing day of the year will be:

1/8W+(D-d) 3/8xTQ MxNA. Where:
W: Weather
D: Debt
d: Money due in January pay
T: Time since Christmas
Q: Time since failed quit attempt
M: General motivational levels
NA: The need to take action

Oh, and "the fact that the most depressing day fell on a Monday was not planned but a coincidence." This really makes me look forward to returning to work next Monday...