Hi,

You could filter the items before return and attach the extra content if certain condition is met like this:

add_filter('stec_event_controller_get_items', function($data, $request) {

    $events = $data->get_data();

    $context = $request->get_param('context');

    // Modify only if data is for the front-end
    if ($context !== 'event') {
        return $data;
    }

    foreach ($events as $k => $event) {

        $condition_is_met = true;

        if ($condition_is_met) {

            $extra_description_content = 'MY EXTRA CONTENT';

            $events[$k]['description'] .= $extra_description_content;

        }

    }

    $data->set_data($events);

    return $data;

}, 10, 2);

Similarly for the event single page. The calendar has following filter:

  $item = apply_filters('stec_event_controller_get_item', $item, $request);