Blue Monday (the first Monday of the year after the holiday season) is considered by many to be the most depressing day of the year.  The term was first coined back in 2005, after psychologist Cliff Arnall created a formula that determined the most depressing day of the year.

But before you decide to call in sick just to stay in bed all day, The Guardian's Pete Etchells is here to explain why this whole thing is nothing but "load of rubbish." He points out:
One recent study, published in December 2013 in the Journal of Affective Disorders, suggests that there's actually no seasonal variation in depression in the general population at all, and any studies that do suggest it might have just overestimated the prevalence.

...

Depression doesn't work like that, and to suggest that it's something minor that everyone goes through from time to time belittles actual clinical depression and makes those who suffer from it acutely aware of how difficult it is to explain to people what they're going through.
[The Guardian]