
While end of the iteration is a good time to do a retrospective and demo/showcase, if your user stories are ready and the product owner is available, it may be useful to do early demos and get the required feedback. Also for retrospectives, if there is a need to deep dive into a specific issue, it would be good idea to do a retrospective with the relevant thinking tools. It's more easier to do it at a structured frequency so you don't skip it and allows sufficient time to handle the actions decided.