C语言，根据输入的字母判断是星期几

你输入了S后的回车还在缓冲区中，后来被读取到letter中了，而不是你输入的字母，所以才会出错，你在之前再加一个getchar就行了。还有你的第二个else if中不需要再读取字母了，直接判断就行了。下面是整理后的代码。我建议你直接读取一段作为字符串来判断比较方便，这样比较麻烦。
#include <stdio.h>
void main()
{
printf("please enter the first letter of someday:\n");
char letter;

while ((letter = getchar()) != 'y') {
switch (letter) {
case 'S': {
printf("please enter the second letter:");
getchar();

if ((letter = getchar()) == 'a') {
printf("It is Saturday!\n");
} else if (letter == 'u') {
printf("It is Sunday!\n");
} else {
printf("error\n");
}

break;
}
case 'M':
printf("It is Monday!\n");
break;
case 'T': {
printf("please enter the second letter:");
getchar();

if ((letter = getchar()) == 'h') {
printf("It is Tuesday!\n");
} else if (letter == 'u') {
printf("It is Thursday!\n");
}

break;
}
case 'F':
printf("It is Friday!\n");
break;
case 'W':
printf("It is Wednesday!\n");
break;
}
}
}

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