content-on-this-day-switch-date-form.php

<?php
$the_month = get_query_var( 'zah-on-this-month' );
$the_day = get_query_var( 'zah-on-this-day' );
$months = array(
	'January',
	'February',
	'March',
	'April',
	'May',
	'June',
	'July',
	'August',
	'September',
	'October',
	'November',
	'December',
);
$days = range(1, 31);

wp_enqueue_script( 'on-this-day' );
?>

<form class="on-this-day-date-switcher" action="<?php echo esc_url( get_site_url() ); ?>/on-this-day/">
	<label class="hidden" for="zah-on-this-month">Select a month</label>
	<select name="zah-on-this-month" id="zah-on-this-month">
		<?php foreach ( $months as $index => $m ) :
			$val = $index + 1;
			if ( $val < 10 ) {
				$val = '0' . $val;
			}
		?>
			<option value="<?php echo esc_attr( $val ); ?>" <?php selected( $val, $the_month ); ?>><?php echo $m; ?></option>
		<?php endforeach; ?>
	</select>
	<label class="hidden" for="zah-on-this-day">Select a day</label>
	<select name="zah-on-this-day" id="zah-on-this-day">
		<?php foreach ( $days as $num ) :
			$val = $num;
			if ( $val < 10 ) {
				$val = '0' . $num;
			}
		?>
		<option value="<?php echo esc_attr( $val ); ?>" <?php selected( $val, $the_day ); ?>><?php echo $num; ?></option>
	<?php endforeach; ?>
	</select>

	<input type="submit" class="submit" value="Try Another Date">
</form>