You can access which value has been submitted by getting the value of $_POST['month']
For Example:
if (isset($_POST['month']) && $_POST['month'] != 'date') {
// add your code here
}
You also need to close your option tags as so:
<div class="formblock">
<select name="month">
<option value="date">Month</option>
<option value="1">January</option>
<option value="2">February</option>
<option value="3">March</option>
</select>
<?php if($emailError != '') { ?>
<br /><span class="error"><?php echo $emailError;?></span>
<?php } ?>
</div>