#define DAYOFMONTH 13
#define DAYOFWEEK 5

#if DAYOFWEEK==5 && DAYOFMONTH==13
It's Friday the 13th!
#endif

#if defined(DAYOFYEAR)
I know what year it is!
#elif DAYOFWEEK>5
Done know the year, but it's not a work day.
#else
Working and no idea what year it is.
#endif

#if DAYOFWEEK*DAYOFMONTH%23==1
Have no idea why this is interesting.
#else
Still have no idea why this is interesting.
#endif