This is the 3689th most frequent Japanese word.
いつぞや
"Once upon a time" or "some time ago."
いつぞや refers to a non-specific, somewhat nostalgic time in the past during which the festival occurred.
いつぞやの祭りでは、たくさんの人が集まっていました。
At some festival from the past, many people gathered.
いつぞや signifies an unspecified instance in time when the speaker visited this mountain.
いつぞや行ったことのある山に、また登りたいです。
I want to climb the mountain I visited once again in the past.
いつぞや highlights an indistinct point in the past when the statement was made.