My assessment is next week, when I should do the demo test with IRIS?

It is strongly recommended to go through demo test at least a week before your actual test. It is also recommended that you run the demo test on the same machine/computer you will sit in actual test. This allows you to test your webcam, microphone and audio in order to smoothly run the actual test. You will also have sufficient time to troubleshoot or report any possible technical issues you might encounter.