Ajax calender extender by default doesn't support this facility. Either you have to change the implementation of AJAX Calender extender or use normal ASP.NET Calender control as below :
protected void calEnglishStartDate_DayRender(object sender, DayRenderEventArgs e)
{
if(e.Day.Date.Date< DateTime.Now.Date)
{
e.Day.IsSelectable = false;
e.Cell.ToolTip = "Cannot select past date.";
}
}