HELP! Birthday Calculator, C language

Clarifications

I'd recommend having CalculateDateDay return a value. e.g

return day;

as the last statement in the function.

Remember you are passing in the value of day, not a reference to where day is stored.
So day becomes a local variable to the function and any changes to it are binned as soon as the function returns.

There are several problems. The reason you print the same day is that you are not calling a function that actually calculates the day. It would be helpful to you and others if you documented your code.

Look at your main function:

int main ()
{
int day;
int month;
int year;
int weekday;

printf("Enter a day of month (1-31):");
scanf("%d", &day);
printf("Enter a month (1-12):");
scanf("%d", &month);
printf("Enter a year:");
scanf("%d", &year);

printDateDay(day, month, year);
}

You now have to look at the function "printDayDate" because it is the only function you are calling:

void printDateDay (int day, int month, int year)
{

printMonth(month);
printf(" %d, ", day);
printf("%d, is a ", year);
printWeekday(month, day, year);

}

printMonth is ok although I would add some bounds checking; what happens if a user enters a number < 1 or > 12?

Now on to printWeekday:

void printWeekday (int month, int day, int year)
{

int weekday = (calculateDateDay(month, day, year)) % 7;

switch(weekday)
{
case 0: printf("Sunday");
break;
case 1: printf("Monday");
break;
case 2: printf("Tuesday");
break;
case 3: printf("Wednesday");
break;
case 4: printf("Thursday");
break;
case 5: printf("Friday");
break;
case 6: printf("Saturday");
break;
}
}

What are you doing as the first function? Calling calculateDateDay, so look there:

int calculateDateDay (int month, int day, int year)
{

int dec31 = ((year - 1) * 365 + (year - 1) / 4 - (year - 1) / 100 + (year - 1) / 400 ) % 7;

switch(month)
{
case 12: day + 30;
case 11: day + 31;
case 10: day + 30;
case 9: day + 31;
case 8: day + 31;
case 7: day + 30;
case 6: day + 31;
case 5: day + 30;
case 4: day + 31;
case 3: day + 28;
case 2: day + 31;
case 1: day + 0 + dec31;
}
}

Look at the function 'dec31'.

I do not know just what you think you are doing, but look closely at that formula.

Lets plug a few numbers in now: oh say 10/31/2008.

s0 == 2007 * 365;
s1 == 2007 / 4;
s2 == 2007 / 100;
s3 == 2007 / 400
dec31 == s0 + s1 -s2 + s3 % 7;
dec31 == 732555 + 733056 - 20 / 5; == 293,118.2
dec31 == dec31 % 7 == 2;

Do you see the problem? Try it with a few other numbers. This is called a walkthrough.

Your switch statement also has a problem.

Also, look closely at your syntax.

We here at techrepublic do not do homework. In your case I made an exception. Your instructor and fellow students are much better resources.