int num = 41; //第几周
string year = "2014"; //年份

DateTime dt;
DateTime.TryParse(year + "-1-1", out dt); //取该年第一天

int dw = Convert.ToInt32(dt.DayOfWeek.ToString("d")); //该年第一天为星期几
int days = (num - 1) * 7 - dw;

DateTime start = dt.AddDays(days);　　//第n周起始日期
DateTime end = dt.AddDays(days + 6); //第n周结束日期

Response.Write(start.ToShortDateString() + " " + end.ToShortDateString());

根据周查出对应的时间段