A report at the link below says, “today is 'saddest day of the year' -- and there's a 'Blue Monday' equation that 'explains' why:
Wintry weather. Post-holiday debt. Failed New Years resolutions. And everyone back in the office after taking time off.
Those things added together amount to what is believed to be the most depressing day of the year for many: today, January 19, dubbed “Blue Monday.”
For math nerds, the actual equation is shown at the second link below.