function Calendar()
{   this.WTBL = ["日","月","火","水","木","金","土"];
    this.MTBL = [0,31,59,90,120,151,181,212,243,273,304,334];

    // 通算日数を計算する関数
    this.Days = function(yy,mm,dd)
    {   var y= parseInt(yy);
        var m= parseInt(mm);
        var d= parseInt(dd);
        var w= (y-1)*365;           //年*３６５
        w+= Math.floor(y/4) - Math.floor(y/100) + Math.floor(y/400) + this.MTBL[m-1] + d;
        if (m<3 && this.Uruu(y))    w--;
        return(w);
    }
    // 閏年を調べる関数
    this.Uruu = function(y)
    {   if (y%400==0)  return(true);
        if (y%100==0)  return(false);
        if (y%4==0)    return(true);
        return(false);
    }
    // 曜日を調べる関数
    this.Week = function(num)
    {   var n= (num)%7;
        return this.WTBL[n];
    }

    var date= new Date();
    var yy= date.getFullYear();
    var mm= date.getMonth()+1;
    var dd= date.getDate();
    this.num= this.Days(yy,mm,dd);
}

<script type="text/javascript">
    var str = prompt("記念日をタイプして下さい", "1995/1/17");
    ymd = str.split('/');
    var cls = new Calendar();
    var n = cls.Days(ymd[0],ymd[1],ymd[2]);
    var cnt = cls.num - n + 1;
    var wek = cls.Week(n);
    alert("記念日 " + str + "(" + wek + "曜日)" + "  経過日数:" + cnt);
</script>