Using an option, I can specify whether even or odd days are valid. For the user interface I search now for suitable symbols to display these options. In mathematics there is no symbol for even and odd numbers, as far as I know.
Does anyone know whether there is perhaps something corresponding in Unicode?
The answer is no.
Given that odd and even numbers are a mathematical concept and mathematics has no symbol for odd and even numbers, maybe except for 2N and 2N+1, you'll find it hard to find a non-existent symbols in Unicode.
You'd have to think of your own characters, or find some in Unicode and just redefine their meaning.