カレンダーのプラグイン

WordPress 2.9.1

functions.php に

function my_get_calendar(){
  ob_start();
  get_calendar();
  $output = ob_get_contents();
  ob_end_clean();

  if(preg_match_all('@<td><a href="' . get_option('home') . '/(\?m=)?([^"]*?)" title="([^"]*?)">[0-9]+?</a>@i', $output, $matches)){
    foreach($matches[0] as $key => $match){
      $date  = preg_replace('@/@', '', $matches[2][$key]);
      $year  = substr($date, 0, 4);
      $month = substr($date, 4, 2);
      $day   = substr($date, 6, 2);

      global $wpdb;
      $posts = $wpdb->get_results("SELECT ID, post_title "
        . "FROM $wpdb->posts "
        . "WHERE YEAR(post_date) = '$year' "
        . "AND MONTH(post_date) = '$month' "
        . "AND DAY(post_date) = '$day' "
        . "AND post_type = 'post' AND post_status = 'publish' "
        . "ORDER BY post_date DESC"
      );

      $link = '';
      foreach($posts as $post)
        $link .= '<a href="'. get_permalink($post->ID) . '">' . $post->post_title . '</a>';

      $output = preg_replace('@' . preg_quote($match) . '@', $match . $link, $output);
    }
  }
  return $output;
}

カレンダーを表示したいところで
echo my_get_calendar();

スタイルは table#wp-calendar 以下の要素をお好みで指定すれば OK。