the ninth day of the month / nine days

the seventh day of the month / seven days

“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

historic, historical / already-happened, past / former, old, traditional

historian

study of history

(Internet slang, especially anime and manga fandom) events or things from the past which one wants to whitewash/erase from history (due to it being embarrassing, poorly received, etc.); dark past

oral history

historicism

Short for 日曜日 (nichiyōbi): Sunday