五月病

Etymology
Literally “May disease” – the Japanese school year and financial year (including new hires) starts in April, thus by May, after the initial period, new students or new hires may be feeling a let-down or depression.

This is also associated with the change of seasons (spring to summer) – compare – and with the main public holiday season at the end of April and the start of May of each year (Golden Week), specifically returning from holiday and re-adapting to work.

Noun

 * 1) Depression sometimes seen in new hires or new students; loosely, “(start of) summer fever”.