<?php
defined('_JEXEC') or die;
 
$dateTime = \JFactory::getDate('now', 'GMT');
$lastMonday = clone $dateTime->modify(
 $dateTime->format('l') == 'Sunday' ? 'Monday last week' : 'Monday this week'
);
$lastMonday = $lastMonday->toSql();

$db = \JFactory::getDbo();
$query = $db->getQuery(true)
->select($db->qn('registerDate'))
->from($db->qn('#__users'))
->where($db->qn('block') . '= 0')
->where($db->qn('registerDate') . '>=' . $db->q($lastMonday))
;

$db->setQuery($query);
$registerDates = $db->loadObjectList();

$days = array(
 'Monday' => 0,
 'Tuesday' => 0,
 'Wednesday' => 0,
 'Thursday' => 0,
 'Friday' => 0,
 'Saturday' => 0,
 'Sunday' => 0,
);

foreach ($registerDates as $Date)
{
 $day = date('l', strtotime($Date->registerDate));
 $days[$day]++;
}

$html = array();
$html[] = '<ul>';

foreach ($days as $day => $count)
{
 $html[] = '<li>' . JText::_(strtoupper($day)) . ': ' . $count . '</li>';
}
$html[] = '</ul>';

echo implode("\n", $html);