Output calendar for any year after 1900

Question: Output the calendar for any year after 1900. The original title is from "The Science and Art of C Language" P132

/*  File name:calendar.c
    Function:generate a calendar for a year
    Time:2018.04.08.00:02
    the source code from book《The art and science of C》,eric roberts
    edited by qjx  
*/

#include <stdio.h>

#define Sunday 0    //easy to qiumo(%)
#define Monday 1
#define Tuesday 2
#define Wednesday 3
#define Thursday 4
#define Friday 5
#define Saturday 6

void GiveInstruction(void);        //tell the input rules
int GetYearFromUser(void);
void PrintCalendar(int year);
void PrintCalendarMonth(int month,int year);    //the essence of this program
void IndentFirstLine(int weekday); //The spacebar of the first row of every month
char *MonthName(int month); //this function as string(char) function can transmit string to display
int MonthDays(int month,int year); //transmit the days of every month
int IsLeapYear(int year); //judge the input number is leapyear or not

main()
{
    int Year;
    GiveInstruction();
    Year=GetYearFromUser();
    PrintCalendar(Year);
}

void GiveInstruction(void)    //tell the user how to do
{
    printf("This program displays a calendar for a full\n");
    printf("year. The year must not be before 1900.\n");
}

int GetYearFromUser(void)
{
    int year;
    while(1){
        printf("Which year?");
        scanf("%d",&year);
        if(year>1900) return (year); //while the input is right,function will return
        printf("The year must be at least 1900.\n"); //otherwise,waring and input again
    }
}

void PrintCalendar(int year)
{
    int month;
    for(month=1;month<=12;month++)
    {
        PrintCalendarMonth(month,year); //print the every month is the essence of program
        printf("\n");
    }
}

void PrintCalendarMonth(int month,int year)
{
    int weekday,nDays,day;
    printf("   %s %d\n",MonthName(month),year); //the head of the monthly calendar
    printf(" Su Mo Tu We Th Fr Sa\n");
    nDays=MonthDays(month,year); //get the days of month,which will call IsLeapYear function
    weekday=FirstDayOfMonth(month,year); //know first day of every month is which one of weekday
    IndentFirstLine(weekday); //print the spacebar before the first day
    for(day=1;day<=nDays;day++)
    {
        printf(" %2d",day); //printf the day
        if(weekday==Saturday) printf("\n");
        //if saturday,line feed;if not,the final weekday will line feed
        weekday=(weekday+1)%7; //calculate next day
    }
    if (weekday!=Sunday) printf("\n");
    //if not sunday,line feed;if sunday,the former weekday is sturday,has already line feed
}

void IndentFirstLine(int weekday) //The spacebar of the first row of every month
{
    int i;
    for(i=0;i<weekday;i++)
    {
        printf("   ");
    }
}

char *MonthName(int month) //transmit the string of months
{
    switch(month)
    {
        case 1:return ("January");
        case 2:return ("February");
        case 3:return ("March");
        case 4:return ("April");
        case 5:return ("May");
        case 6:return ("June");
        case 7:return ("July");
        case 8:return ("August");
        case 9:return ("September");
        case 10:return ("October");
        case 11:return ("November");
        case 12:return ("December");
        default:return ("Illegal month");
    }
}

//calculate the numbers of every month,notice need to call IsLeapYear function
int MonthDays(int month,int year)
{
    switch(month)
    {
        case 2:
            if(IsLeapYear(year))
                return 29;
            return 28;
        case 4: case 6: case 9: case 11:
            return 30;
        default:
            return 31;
    }
}

int FirstDayOfMonth(month,year) //know first day of every month is which one of weekday
{
    int weekday,i;
    weekday=Monday;
    for(i=1900;i<year;i++) //first according to the year
    {
        weekday=(weekday+365)%7; // % can get the weekday
        if(IsLeapYear(i))  weekday=(weekday+1)%7;
    }
    for(i=1;i<month;i++) //second according to the month
    {
        weekday=(weekday+MonthDays(i,year))%7;
    }
    return weekday;
}

int IsLeapYear(int year) //judge the input number is leapyear or not
{
    return ((year%400==0)||((year%4==0)&&(year%100!=0)));
}