Hi,
Try this:
<?php
$datetime_string = get_field('fecha_del_evento');
$datetime = new DateTime(date('Y-m-d', strtotime($datetime_string)));
echo $month = date_i18n('M', strtotime($datetime_string));
echo $day = $datetime_object->format("d");
?>
It is possible that you need to change the way you save the date values in DB. d/m/Y is really not optimal for datetime interactions and you should save it as Ymd.
