I would expect the code you posted above to produce an output of "Day ", because you didn’t provide any formatting markers in the format specifier string (which was "Day "); the end result being you get out exactly what went in.
For format to work, it needs a format specifier to tell it where to place the arguments you’ve provided to it. It doesn’t just blindly concatenate things together in the order you passed them.
My answer goes into more detail about how format specifiers can be set-up.