this-week.php

<?php
	$first_date = "{event_last_date format="%Y%m%d"}";
	$next_week = date( "Ymd", strtotime( "+1 Sunday 23:59" ) );
	$today = date( "Ymd", strtotime( "now" ) );


	echo "<p>First date: " . $first_date . "<br>";
	echo "next_week: " . $next_week . "<br>";
	echo "today: " . $today . "</p>";

	if ( 
		intVal( $today ) <= intVal( $first_date ) 
		AND intVal( $first_date ) <= intVal( $next_week ) 
	) {
		echo '<p>Add this class</p>';
	}
?>