Add body class to events with a particular category

example.php

<?php // <== dont copy this line if your functions.php file already has the php opening tags

add_filter('body_class','my_add_custom_body_class');
function my_add_custom_body_class($classes) {

    if( is_singular('tribe_events') && has_term( 'freeevents', 'tribe_events_cat' ) ) {
        $classes[] = 'is-free-event';
    }

    return $classes;
}