const __flash char *Illegal = {"Illegal month"}; 
const __flash char *Januar  = {"January"};
.....
const __flash char *Decab  = {"December"};
//не даст - попробуй так: const __flash char Illegal[] = {...};
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а потом вне функций:
const __flash char * __flash Month[]  = 
{
 Illegal,
 Januar,
 .....
 Decab
};