我觉得我这个比较严谨嘿嘿

#include <stdio.h>
int main() {
int n;
do {
scanf("%d", &n);
if (n < 0) {
printf("请输入大于零的数");
}
} while (n < 0);
while (n >= 7) {
n -= 7;
}
switch (n) {
case 0:
printf("monday");
break;
case 1:
printf("tuesday");
break;
case 2:
printf("wednesday");
break;
case 3:
printf("thursday");
break;
case 4:
printf("friday");
break;
case 5:
printf("saturday");
break;
case 6:
printf("sunday");
break;
}
return 0;
}