请高手诊断为什么会出错！
#include <stdio.h>
void main()
{
char letter;
printf("please input the first letter of someday\n");
while ((letter=getchar())!='Y')/*当所按字母为Y时才结束*/
{
switch (letter)
{
case 'S':printf("please input second letter\n");
if((letter=getchar())=='a')
printf("saturday\n");
else if ((letter=getchar())=='u')
printf("sunday\n");
else printf("data error\n");break;
case 'F':printf("friday\n");break;
case 'M':printf("monday\n");break;
case 'T':printf("please input second letter\n");
if((letter=getchar())=='u')
printf("tuesday\n");
else if ((letter=getchar())=='h')
printf("thursday\n");
else printf("data error\n");
break;
case 'W':printf("wednesday\n");break;
default: printf("data error\n");
}
}
}

当输入S时，显示提示输入第二个字母，当输入a时没有显示saturday，输入T也有同样的情况。我是初学者，请高手帮助

怎样赋值啊!

#include <stdio.h>
void main()
{
char letter;
printf("please input the first letter of someday\n");
while ((letter=getchar())!='Y')/*当所按字母为Y时才结束*/
{
switch (letter)
{
case 'S':printf("please input second letter\n");
getchar();
if((letter=getchar())=='a')
printf("saturday\n");
else if ((letter=getchar())=='u')
printf("sunday\n");
else printf("data error\n");break;
case 'F':printf("friday\n");break;
case 'M':printf("monday\n");break;
case 'T':printf("please input second letter\n");
if((letter=getchar())=='u')
printf("tuesday\n");
else if ((letter=getchar())=='h')
printf("thursday\n");
else printf("data error\n");
break;
case 'W':printf("wednesday\n");break;
default: printf("data error\n");
}
}
}

第一个saturday通过，sunday没通过！

#include <stdio.h>
void main()
{
char letter;
printf("please input the first letter of someday\n");
while ((letter=getchar())!='Y')/*当所按字母为Y时才结束*/
{
switch (letter)
{
case 'S':printf("please input second letter\n");
getchar();
if((letter=getchar())=='a')
printf("saturday\n");
else if (letter=='u')
printf("sunday\n");
else printf("data error\n");break;
case 'F':printf("friday\n");break;
case 'M':printf("monday\n");break;
case 'T':printf("please input second letter\n");
getchar();
if((letter=getchar())=='u')
printf("tuesday\n");
else if (letter=='h')
printf("thursday\n");
else printf("data error\n");break;
case 'W':printf("wednesday\n");break;
default: printf("data error\n");
}
}
}

谢谢横眉冷对，这个程序是完全通过的程序，愿与大家分享。