What is a bygone day?

Excuse me, could you please elaborate on the phrase "bygone day"? I'm curious to know what it means in the context of everyday language. Is it referring to a day that has passed and is no longer relevant, or does it have a deeper metaphorical meaning? Perhaps it's used to signify a time of nostalgia or regret, where we look back on the past with a sense of longing or loss. I'm intrigued to learn more about its origins and how it's commonly used in conversation.