C语言：定义函数给出任意一天对应是星期几

以下是一个例子：

#include <stdio.h>

int dayOfWeek(int y, int m, int d) {
    // Zeller's congruence algorithm
    if (m < 3) {
        y--;
        m += 12;
    }
    int c = y / 100;
    y %= 100;
    int w = (c / 4) - (2 * c) + ((y + (y / 4)) + ((13 * (m + 1)) / 5) + d - 1);
    return ((w % 7) + 7) % 7;
}

int main() {
    int year, month, day;
    printf("Enter the year, month, and day: ");
    scanf("%d %d %d", &year, &month, &day);
    int dayOfWeekNumber = dayOfWeek(year, month, day);
    switch (dayOfWeekNumber) {
        case 0:
            printf("Sunday\n");
            break;
        case 1:
            printf("Monday\n");
            break;
        case 2:
            printf("Tuesday\n");
            break;
        case 3:
            printf("Wednesday\n");
            break;
        case 4:
            printf("Thursday\n");
            break;
        case 5:
            printf("Friday\n");
            break;
        case 6:
            printf("Saturday\n");
            break;
    }
    return 0;
}

在这个例子中，我们使用了 Zeller's congruence 算法来计算星期几。用户输入年、月、日后，我们调用 `dayOfWeek` 函数来得到星期几的数字，然后使用 `switch` 语句来打印对应的星期几字符串。