Combobox and Items!

private void Form1_Load(object sender, EventArgs e)
{
    int[] years = Enumerable.Range(DateTime.Now.Year-100, 101).OrderByDescending(x=>x).ToArray();
    cbYear.DataSource = years;
    string[] month = { "январь", "февраль", "март", "апрель", "май", "июнь", "июль", "август", "сентябрь", "октябрь", "ноябрь", "декабрь" };
    cbMonth.DataSource = month;
    cbMonth.SelectedIndexChanged += YearOrMonth_Changed;
    cbMonth.SelectedIndex = DateTime.Now.Month - 1;
    cbYear.SelectedIndexChanged += YearOrMonth_Changed;
}
 
void YearOrMonth_Changed(object sender, EventArgs e)
{
    DateTime dt = new DateTime(int.Parse(cbYear.SelectedItem.ToString()), cbMonth.SelectedIndex + 2, 1).AddDays(-1);
    var days = Enumerable.Range(1, dt.Day).ToArray();
    cbDay.DataSource = days;
}

int year = Convert.ToInt32(Year.SelectedItem);
int month = Month.SelectedIndex + 1;
 
int daysInMonth = DateTime.DaysInMonth(year, month);
Day.DataSource = Enumerable.Range(1, daysInMonth).ToArray();