Calendar - current week
Copy template file
sites/all/modules/calendar/theme/calendar-month-row.tpl.php
to
sites/all/themes/YOUR_THEME_NAME/templates directory and edit it in this manner:

&lt;?php
/**
 * @file
 * Template to display a row
 * 
 * - $inner: The rendered string of the row's contents.
 */
$attrs = ($week &gt; 0) ? 'week="' . $week . '"' : '';
$attrs .= ($iehint &gt; 0) ? ' iehint="' . $iehint . '"' : '';
if ($week == date('W')) $class .= ' calendar-current-week';
$attrs .= ($class) ? ' class="' . $class . '"': '';
?&gt;
&lt;?php if ($attrs != ''):?&gt;
&lt;tr &lt;?php print $attrs?&gt;&gt;
&lt;?php else:?&gt;
&lt;tr&gt;
&lt;?php endif;?&gt;
  &lt;?php print $inner ?&gt;
&lt;/tr&gt;


Don't forget to clear page cache after this.