- date_create: documentation ( source)
<?php
$checkday = "Sunday";
$period = "2 Days";
$dtStart = date_create('Tomorrow')->modify('last '.$checkday);
$dtEnd = (clone $dtStart)->modify($period);
$dtToDay = date_create('today');
if($dtToDay >= $dtStart AND $dtToDay < $dtEnd){
echo 'run function';
}
else {
echo 'do nothing';
}