Stopping an event from happening

Hi i have the following method which fills up a textbox based on a dropdown;

  protected void ddlMonths_SelectedIndexChanged(object sender, EventArgs e)
{
string strmonth = ddlMonths.SelectedItem.Text.ToUpper();

if (ddlMonths.SelectedItem.Text.ToUpper() == strmonth)
{
CalendarExtenderStartDate.SelectedDate = DateTime.Parse(strmonth);
}

}
 I can the change this textbox value by using the popup calendar, below the textbox i also have a dropdownlist which calculates the price, my problem is on page load the textbox reverts back to the value set in the above method, this happens when the price dropdownlist is selected, here is the method calling it;
protected void ddlLength_SelectedIndexChanged(object sen ...