Add data attribute to each 'door'.

templates/content-advent_day.php

-<div class="calendar_day_cell">
-	<h2 <?php post_class('middle') ?>>
+<div class="calendar_day_cell" data-day-name="<?php echo esc_attr( $post->post_name ); ?>">
+	<h2 <?php post_class( 'middle' ) ?>>
 		<?php the_title(); ?>
 	</h2>
 
@@ -8,6 +8,7 @@
 	$video   = get_field( 'acd_video' );
 	$alt     = get_field( 'acd_alt_description' );
 	$alt_tag = ! empty( $alt ) ? sprintf( 'alt="%s"', esc_attr( $alt ) ) : '';
+
 	if ( isset( $image ) && $image['id'] ) {
 		?>
 		<div class="day-image hidden">
@@ -21,9 +22,11 @@
 		</div>
 		<?php
 	}
+
 	$motto         = get_field( 'acd_short_message' );
 	$joke_question = get_field( 'acd_joke_question' );
 	$joke_answer   = get_field( 'acd_joke_answer' );
+
 	switch ( get_field( 'acd_motto_or_joke' ) ) {
 		case 'motto':
 			?>