noun, noun which may take the genitive case particle 'no'
past days; bygone days; days of yore
Literary expression for a time that has passed, often with a nostalgic or poetic tone. Used as a noun or with の to modify another noun.
在りし日の思い出がよみがえる。
Memories of bygone days come back to me.
在りし日の彼の姿を思い浮かべる。
I recall how he looked in days gone by.
noun which may take the genitive case particle 'no', noun
while still alive; during one's lifetime
Refers to the time when someone was alive; often used in reminiscence about a deceased person. Can be used as a noun or with の.
在りし日の祖父はよくこの公園を散歩していた。
My grandfather, in his lifetime, often took walks in this park.
在りし日の母の言葉が今も心に残っている。
My mother's words from when she was alive still remain in my heart.
昔 is a general, everyday word for 'old times' or 'long ago'. 在りし日 is more literary and nostalgic, often implying a personal or emotional connection to the past.
生前 is a neutral, factual term for 'during one's lifetime', commonly used in legal or biographical contexts. 在りし日 carries a poetic, elegiac nuance when referring to a deceased person.
From the archaic verb 在り (ari, 'to be, to exist') + the classical past auxiliary verb し (shi) + 日 (hi, 'day'). The phrase literally means 'the days that were' and is used in modern Japanese as a set literary expression.