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add_filter( 'easy_booking_calendar_event_description', 'add_custom_event_description', 10, 3 );

function add_custom_event_description( $description, $order_item, $order ) {

    $description = '<p>Client: ' . $order->get_formatted_billing_full_name() . '</p>';
    $meta_data = $order_item->get_formatted_meta_data();

    if ( ! empty( $meta_data ) ) {

        foreach ( $meta_data as $meta_id => $data ) {
            $description .= '<p>' . $data->display_key . ': ' . $data->value .'</p>';
        }

    }

    return $description;

}