How do I make the first option in a drop down list an unacceptable answer?

Question

Okay so I have a form and within this form are a bunch of required fields. The birthday part of this form consists of 3 fields: month, day, and year. Now what I want to do is have the words "Month:", "Day:", and "Year:" at the top of the drop down lists, BUT I do not want these options to be acceptable answers! When I submit the form without selecting an actual month, day, and year, it should say that the month, day, and year are required when I print the errors! How can I accomplish that? My "if statements" that make the fields required are at the bottom of this code:

<form action="" method="post">  
        <ul id="register">
            <li>
                Birthday:

                <select name="month">
                    <option value="month">Month:</option>
                    <option value="January">January</option>
                    //all the other months
                </select>

                <select name="day">
                    <option value="day">Day:</option>
                    <option value="1">1</option>
                    //all the other days of the month
                </select>

                <select name="year">
                    <option value="year">Year:</option>
                    <option value="2013">2013</option>
                    //more year options
                </select><br><br>
            </li>

            <li>
                <input type="submit" name="registrationform" value="Sign up">
            </li>
        </ul>
    </form>
    <?php
    if (!empty($_POST['registrationform'])) {
    if(isset($_POST['registrationform'])){
        $required_fields = array('month', 'day', 'year');
        foreach ( $required_fields as $key=>$value) {
         if (!isset($_POST[$value]) || $_POST[$value]=='') {
              $errors[$value] = $key." is required";

            }
   }

   print_r($errors);
}
    }

    ?>

Answer 1

Use optgroup

<select name="month">
  <optgroup label="Month:">
    <option value="January">January</option>
    //all the other months
  </optgroup>
</select>

etc.

JSFiddle

or disable the option and select the title if you want the title to show - once the list is touched, the title will not be selectable.

<select name="month">
    <option disabled="disabled" selected="selected">Month:</option>
    <option value="January">January</option>
    //all the other months
</select>

JSFiddle