public static DateTime FirstDayOfWeek(DateTime date)
{
    DayOfWeek fdow = CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek;
    int offset = fdow - date.DayOfWeek;
    DateTime fdowDate = date.AddDays(offset);
    return fdowDate;
}

public static DateTime LastDayOfWeek(DateTime date)
{
    DateTime ldowDate = FirstDayOfWeek(date).AddDays(6);
    return ldowDate;
}