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