<?php
$week_day = [
1 => 'ПН', 2 => 'ВТ', 3 => 'СР', 4 => 'ЧТ', 5 => 'ПТ', 6 => 'СБ', 7 =>'ВС'
];
foreach ($week_day as $key => $day) { ?>
<div class="item <?php echo ($key == wp_date('N') ? 'current_day' : '') ?>"><?php echo $day; ?></div>
<?php } ?>