Planday's API are hosted in two distinct regions in the world: Europe and Australia. To reduce end user latencies, Planday operate a CDN and by default route your HTTP request to the fastest region available to you.
To learn which region was automatically chosen for your request, check the x-Backend-Region header of the response to see whether it was routed to westeu or aueast. This applies to all requests except for token exchange which does not contain this header.
Not specifying a region can lead to longer response times if your service is sending requests, for instance, from Europe but tries to reach a portal in Australia. In such cases, you would benefit from specifying the target region where your request should be routed. To learn where a portal is located, check its country using the GET portal information endpoint.
You can specify the region where your request should be routed by adding a X-OpenAPI-Region header to your HTTP request and specifying either westeu or aueast as its values.