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I haven't got an event module set up in D6 but have for D5, so I manually inserted the code from the patch, had to change one line

    return l((int)$date['day'], 'event/'. _event_format_url($date) .'/day', array('attributes' => array('title' => $eventlist)) );

to

   return l((int)$date['day'], 'event/'. _event_format_url($date) .'/day', array('title' => $eventlist));

This is due to the change in usage of l() in D6

The patch works fine, nice feature!

patch works for 6.x 2.x-dev

I am not convinced this is a great idea: If you have more than very few events on the same day, you'll get a very long title attribute.

What if we put limit like this:

function event_render_day_single($date, $view, $types, $terms, $rewrite_parameter = array()) {
  $nodes = event_calendar_data($date, $view, $types, $terms, 'lookup', array(), $rewrite_parameter);
  if (count($nodes)) {
    $eventlist = array();
    foreach ($nodes as $node) {
        $eventlist[] = $node->title;
    }
    $eventlist = implode(" / ", $eventlist);
    if (strlen($eventlist) > 100) {
      $eventlist = substr($eventlist, 0, 100) . ' ...';
    }
    return l((int)$date['day'], 'event/'. _event_format_url($date) .'/day', array('attributes' => array('title' => $eventlist)) );
  }
  else {
    return (int)$date['day'];
  }
}

Event for Drupal 8 is unrelated to older versions. If an issue similar to this one exists, please open a new issue with the 8.x branch.