This phrase is often used to describe:
* The passage of time: The days, like a song, have their own rhythm and melody, and the phrase evokes the feeling of time flowing by.
* The beauty of everyday life: The "song" might be a reflection of the experiences, joys, and sorrows that make up each day, creating a unique melody.
* A poetic or philosophical perspective on life: It suggests that life itself is a beautiful and intricate song, made up of countless individual days.
The exact meaning can vary depending on the context and the author's intentions.
Do you have a specific example of where you encountered this phrase? I can give you a more tailored explanation if you let me know the context.