string[] Day = new string[] { "星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六" };
string week = Day[Convert.ToInt32(DateTime.Now.DayOfWeek.ToString("d"))].ToString();
lbDay.Text = week;

lbDay.Text = System.Globalization.CultureInfo.CurrentCulture.DateTimeFormat.GetDayName(DateTime.Now.DayOfWeek);

public static string dt;
public static string week;
dt = DateTime.Today.DayOfWeek.ToString();
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;
}
lbDay.Text = week;