You can catch the CurrentCellShowingDropDown event and set the Maxdate and Mindate on the embedded MonthCalendar at that point.
private void gridControl1_CurrentCellShowingDropDown(object sender,
GridCurrentCellShowingDropDownEventArgs e)
{
GridDropDownMonthCalendarCellRenderer cr = this.gridControl1.CurrentCell.Renderer
as GridDropDownMonthCalendarCellRenderer;
if(cr != null)
{
MonthCalendar calendar = cr.DropDownContainer.Controls[0] as MonthCalendar;
calendar.MaxDate = new DateTime(2005, 8, 31);
calendar.MinDate = new DateTime(2005, 7, 1);
}
}