#include <stdio.h>

int get_ymd(const char *s, int *yp, int *mp, int *dp)
{
    int n;  char c;

    n = sscanf(s,  "%4d%2d%2d %c", yp, mp, dp, &c);
    if (n == 3) return 1;
    n = sscanf(s,  "%d %*c%d %*c%d %c", yp, mp, dp, &c);
    if (n == 3) return 1;
    return 0;
}

char *week[] = {
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
};

char *month[] = { "",
    "Jan", "Feb", "Mar", "Apr", "May", "Jun",
    "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
};

int dow(int y, int m, int d)
{
    static char t[] = { 0, 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 };
    y -= m < 3;
    return (y + y/4 - y/100 + y/400 +t[m] + d) % 7;
}

int main(int argc, char **argv)
{
    int y, m, d, i;  char buf[1024], *p = buf;

    for (i = 1; i < argc; i++)
        p += sprintf(p, " %s", argv[i]);
    if (get_ymd(buf, &y, &m, &d) && m>=1 && m<=12)
        printf("%s %s %d %d\n", week[dow(y, m, d)], month[m], d, y);
    return 0;
}

"%d"   0個以上の空白を読み飛ばし、10進数を読み込む。

" "    0個以上の空白を読み飛ばす。

"%*c"  1文字読み飛ばす。

  m =  0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12

t[] = {0, 6, 2, 1, 4, 6, 2, 4, 0, 3,  5,  1,  3};