Mus musculus, the common house mouse

happening once a year

“a hundred days of sermonizing, one fart” → a metaphor for how something that one has put great effort into can come to nothing after even a slight mistake, much as a hundred days of serious spiritual study can be lost to the winds

year

a grade, a school year / a year

years

short span of time

one, 1 / the first in order, foremost / the beginning / the best / a kind of; one of something / (card games) an ace / unity / the bottom string of a shamisen or other Japanese musical instrument

nth-year student or pupil

each person