Output details just before the ticket selector, this hook passes the EE_Event ($event) for use within the function if needed.

display_message_before_the_ts.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file
//Output details just before the ticket selector, the function has the EE_Event object ($event) available for use.

function tw_output_details_before_the_ts( $event ) {

	echo "<h1>TESTING TESTING</h1>";

}
add_action( 'AHEE__ticket_selector_chart__template__before_ticket_selector', 'tw_output_details_before_the_ts' );