给你一个简单的事例：

    public static string dt;
    public static string week;
    protected void Button1_Click(object sender, EventArgs e)
    {
        //获取当前日期是星期几
        dt = DateTime.Today.DayOfWeek.ToString();
        //根据取得的英文单词返回汉字
        switch (dt)
        { 
            case "Monday":
                week ="星期一";
                break ;
            case "Tuesday":
                week = "星期二";
                break;
            case "Wednesday":
                week = "星期三";
                break;
            case "Thursday":
                week = "星期四";
                break;
            case "Friday":
                week = "星期五";
                break;
            case "Saturday":
                week = "星期六";
                break;
            case "Sunday":
                week = "星期日";
                break;
        }
        //利用TextBox文本框显示当前日其为星期几
        TextBox1.Text = week;
    }
}

这个很简单啊，直接用C#的DateTime类就可以了，如下：

// 获取当前日期和时间
 DateTime now = DateTime.Now;
 // 获取今天是星期几
 DayOfWeek dayOfWeek = now.DayOfWeek;