Changes the header for today and tomorrow in lists that are grouped by day.

change_grouped_by_day_header.php

<?php
/**
 * Changes the header for today and tomorrow in lists that are grouped by day.
 *
 * @param	  string  $header   The header.
 * @param	  string  $day	    The day.
 * @param	  array   $args	    The arguments for the HTML of this list.
 */
function change_grouped_by_day_header( $header, $day, $args ) {
  
  if ( date( 'Y-m' ) == $day ) {
    $header = 'Today';
  }
  
  if ( date( 'Y-m', strtotime( 'Tomorrow' ) ) == $day ) {
    $header = 'Tomorrow';
  }
  
  return $header;
  
}

add_filter( 'wpt_listing_group_day', 'change_grouped_by_day_header', 10, 3 );