It's not good writing. There is no authority on language, so "permissible" has nothing to do with it. Writing guidelines are drawn from what makes the communication effective and pleasing to the reader; often this amounts to "everyone else does it this way," which is where the "rules" you learn in elementary school come from. That said, misuse of commas can be very distracting as they can convey a variety of syntactical operations which readers must parse. They are the most distracting punctuation when used in an unexpected manner. In this case the author would do better to use "Every. Single. Day."

/pedantry (few threads seem more appropriate for it)