By using the backslash, you prevent $month from being interpolated in the doublequotes. The shell then gets $month and tries to expand it - but there is no shell variable named $month.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Using scalar variable in system set of commands by choroba
in thread Using scalar variable in system set of commands by dtbach

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