yopu would need to put an if statement around the select list to determine if ot should be shown
you say you have LastTimeDate column that has the last day to show the event? the something like this should do it
<?php
do {
?>
<?php if($row_ClassDetails['LastTimeDate'] >= date("Y-m-d")) { ?>
<option value="<?php echo $row_ClassDetails['CDID']; ?>" ><?php echo $row_ClassDetails['CDDate']; ?> <?php echo $row_ClassDetails['CDName']; ?></option>
<?php } ?><?php
} while ($row_ClassDetails = mysql_fetch_assoc($ClassDetails));
?>