old days; former times; bygone days
Literary or formal term for the distant past, often with a nostalgic or reflective tone. Not used in casual conversation.
昔日の面影はもうどこにもない。
There is no trace left of the old days.
この町には昔日の繁栄をしのばせる建物が残っている。
Buildings that recall the prosperity of bygone days remain in this town.
昔日 is a literary, somewhat archaic word for 'days of old', similar in register to 往年 but less commonly used in modern speech.
Compound of 昔 (olden times) and 日 (day). The reading せきじつ is the on'yomi of the kanji. The exact historical derivation is uncertain, but the word has been used in classical Japanese and Chinese-influenced writing.