The 140 project records how depression is described, by poets and science, over the average length of an episode of major depression. Counting days is a popular method of looking forward to the end of something, and also to connect the present to the past. Thereare lots of examples: the first 100 days, the book of days (1869), and a newer version (2004), Around the World in Eighty Days...
According to this article, the average duration of a major depressive episode is 140 days. Other sources suggest it is 6 months. The reality is its hard to pinpoint the exact day an episode begins or ends, and there's lots of variables involved. What is important: that people know there is an end to the darkness.