Try this
private void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
ddlMonth.DataSource = Enumerable.Range(1, 12).Select(a => new
{
MonthName = DateTimeFormatInfo.CurrentInfo.GetMonthName(a),
MonthNumber = a
}
);
ddlMonth.DataTextField="MonthName";
ddlMonth.DataValueField="MonthNumber";
ddlMonth.DataBind();
ddlYear.DataSource = Enumerable.Range(DateTime.Now.Year - 99, 100).Reverse();
ddlYear.DataBind();
ddlday.DataSource = Enumerable.Range(1, DateTime.DaysInMonth(DateTime.Now.Year, Convert.ToInt32(ddlMonth.SelectedValue)));
ddlday.DataBind();
}
}