Create Solspace Calendar Event on Commerce onOrderComplete
To listen to Craft Commerce order save event, you must make a new plugin (or use an existing one) and in it's ::init() method, listen to the commerce_orders.onSaveOrder event:

class TestPlugin extends BasePlugin
{
    public function init()
    {
        parent::init();

        craft()-&gt;on(
            "commerce_orders.onSaveOrder",
            function (Commerce_OrderModel $order) {
                // Create the event
                $event = new Calendar_EventModel();

                $event-&gt;getContent()-&gt;title = "New Event Title";
                $event-&gt;slug                = "new-event-title";

                // Any custom fields go through the ::getContent() model
                $event-&gt;getContent()-&gt;body = "This is a custom rich text field";

                // Do whatever you need to do with the order data
                // By using the $order OrderModel
                // For example:
                //
                // $event-&gt;getContent()-&gt;body = "Order total: " . $order-&gt;totalPrice;

                // Mandatory fields
                $event-&gt;calendarId = 1;
                $event-&gt;authorId   = craft()-&gt;getUser()-&gt;id;
                $event-&gt;startDate  = new DateTime();
                $event-&gt;endDate    = new DateTime();

                $event-&gt;allDay = true;

                // Save the event using Calendar Events service ::saveEvent() method
                craft()-&gt;calendar_events-&gt;saveEvent($event);
            }
        );
    }
}