struct date today,term;
int main()
{   int yearday,term_year,day_year,days;
    printf("Enter year/month/day:");
    scanf("%d%d%d",&today.year,&today.month,&today.day);
    term.month=12;
    term.day=31;
    yearday=0;
    for(term_year=1991;term_year<today.year;term_year++)
    {
        date_day(day_year);
        yearday+=date_day(day_year);          /*计算从1990年至指定年的前一年共有过少天*/
    }
    yearday+=date_day(today);             /*加上制定年到指定日期的天数*/
    days=yearday%5;                      /*求余数*/
    if(days>0&&days<4)/*将day改成days*/
    printf("He was fishing at that day.\n");
    else
    printf("He was sleeping at that day.\n");
}

命名上有错误  不能用点

#include<stdio.h>
struct date
{
    int year;
    int month;
    int day;
};
int date_day(int day_year)
{
    static int day_tab[2][13]=
    {{0,31,28,31,30,31,30,31,31,30,31,30,31,},     /*非闰年*/
    {0,31,29,31,30,31,30,31,31,30,31,30,31,},      /*闰年*/
    };
    int i,lp,day_day,day_month;
    day_month=13;
    day_day=0;
    lp=day_year%4==0&&day_year%100!=0||day_year%400==0;
    for(i=0;i<day_month;i++)
        day_day+=day_tab[lp][i];
    return(day_day);
}
struct date today,term;
int main()
{   int yearday,term_year,day_year,days,xx;
    printf("Enter year/month/day:");
    scanf("%d%d%d",&today.year,&today.month,&today.day);
    term.month=12;
    term.day=31;
    yearday=0;
    for(term_year=1991;term_year<today.year;term_year++)
    {
        xx=date_day(day_year);
        yearday+=xx;          /*计算从1990年至指定年的前一年共有过少天*/
    }
      xx=date_day(day_year);
    yearday+=xx;             /*加上制定年到指定日期的天数*/
    days=yearday%5;                      /*求余数*/
    if(days>0&&days<4)
    printf("He was fishing at that day.\n");
    else
    printf("He was sleeping at that day.\n");
getch();
}
我修改了一下，你不能直接用函数来进行加减，我定义了一个XX,用来装函数的返回值，然后最后if(days>0&&days<4)
你用的是day，我没有仔细看程序，因为你定义的变量都没有注解的，有点乱。我认为是days这个参数，直接改为days，因为day没有定义，就是说这个变量不知道从哪里来的。还有就是函数定义你应该给定他数据类型。我编译成功了。因为没有完全看懂程序，按大概意思改的。

date.day(year)    这个函数有点雷人
 return(day.day); 递归吗我倒  
 int yearday; for(year=1991;year<today.year;year++)
                 声明了吗
。。。。。。。。。