"The day you left me was the day I fell in shambles".?

"fell in shambles" isn't correct. "became a shambles" would be better, or "fell apart"

Your sentence is grammatically correct. The 'why' is because there's nothing grammatically wrong with it.

A "shambles" is a slaughterhouse. Idiomatically, "shambles" is used without an article before it to mean a scene of destruction, disorder and chaos. A Google Ngram found that the phrase "fell in shambles" was never said in any book or any article published in English between 1800 and 2014, which confirms what I already knew: "fell in shambles" isn't idiomatic. Idiomatically, we say that something (not someone) was (not fell) in shambles.

On the other hand, if you are not applying the idiom but speaking literally, you are more than welcome to say, "The day you left me was the day I fell in 'a' shambles," which would mean, "The day you left me was the day I fell in a slaughterhouse."

However, what I think you mean is:

The day you left me was the day I fell to pieces.

-or-

The day you elft me was the day I fell apart.

Basically, it seems as if you are mixing metaphors. You are mixing "fell to pieces" with "was in shambles" to come up with "fell in shambles," which is meaningless. Were I to likewise mix metaphors, I might say that a writer who mixes metaphors is "green behind the ears."

It sounds good to me. Though I would probably say "The day that you left me, was the day I fell in shambles."
It gives that extra flair to it.
The "that" makes the sentence flow smoother, and the comma breaks it into two parts or meanings it symbolizes a small pause just adds a little extra dramatic flair. If you're into that.

Since the day you left me, my whole life has been in shambles.