Custom post type advent-calendar not showing in RSS feed

Solved it with this.

// Add a Custom Post Type to a feed
	function add_cpt_to_feed( $qv ) {
	  if ( isset($qv['feed']) && !isset($qv['post_type']) )
	    $qv['post_type'] = array('post', 'acal_entry');
	  return $qv;
	}

	add_filter( 'request', 'add_cpt_to_feed' );

Got the info from https://yoast.com/dev-blog/custom-post-type-snippets/