The School year starts in April in Japan and fiscal year for business stars also from April. There are various, big, environmental changes between March and April such as graduation, entering school or company, internal transfer and moving for that, etc.
People try very hard to adjust their situation, however, they are getting tired after May, and unbalanced both mentally and physically as experienced symptoms with depressive tendency. It’s called “Gogatsu-Byo” (May Sickness).
There are some people who are absent from a school or company for this sickness.