Hi,
While you could use these date format overrides to handle this, that means that you'll see the week number for all the dates on your website instead of day + month, so that's not really practical.
So you'll have to do this with a view file override.
In the menu Display>Views, you would need to edit the view file "add_to_cart_ajax.php" (for the listings) and "quantity.php" (for the product details page) and change the line:
echo JText::sprintf('ITEM_SOLD_ON_DATE', hikashop_getDate($start_date, $this->params->get('date_format', '%d %B %Y')));
There you can see the date format: %d %B %Y
So change this to remove the day and month and add the week number. As per
www.php.net/manual/en/datetime.format.php
the week number should be available with the letter W