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

fall (into a hole) / sink / to feel sad / be in a slump (e.g. an economic slump)

feeling, mood

a female given name

(archaic) the fifth month of the lunar calendar / (poetic) May / (botany) Rhododendron indicum, a kind of azalea native to Japan (so named because it usually blooms in May)

May

altitude sickness

(by extension from the "thorough" sense) so thoroughly knowledgeable and particular about something as to be annoying to others: picky, nitpicking, fussy / pushy, bossy, overly particular and demanding

moody person

a heavy rain that occurs around the fifth lunar month; an early summer rain / (figuratively) that which repeats over and over without lasting long