DateTime date = DateTime.Now.AddMonths(-1);
DateTime date2 = new DateTime(date.Year, date.Month, 21, date.Hour, date.Minute, date.Second);
label1.Text = date2.ToString();
or alternatively:
DateTime now = DateTime.Now;
DateTime date = now.AddMonths(-1);
int days = DateTime.DaysInMonth(date.Year, date.Month);
DateTime date2 = now.AddDays(-days - now.Day + 21);
label1.Text = date2.ToString();