error: too few arguments to function `printDay’ (C language)

1. What does it mean?

“error: too few arguments to function ‘printDay’” means you’re passing the wrong number of argument to printDay when you call it here:

printDay(input());

You’re passing one argument but your declaration of printDay shows that it takes 3 arguments:

void printDay(int month, int day, int firstDay);

2. How do I fix this?

You can fix it by passing the correct number of arguments, e.g:

int month = ...;
int day = ...;
int firstDay = ...;
printDay(month, day, firstDay);