More specifically, they shift 5 out from each region every week Thursday nights into Friday to add in the next 5 later Friday. They air every region in chronological order, and, save a few exclusives (like the Kyurem vs. Keldeo movie earlier this week that was exclusively shown via the app), the episodes in the app are the same as at
www.pokemon.com/tv-episodes/watch , so if you want to see what's available that week on the app, you can generally go to the official website on your computer and see what's airing on the official website that week, and you can check the upcoming episodes on the website to see if there's anything you want to watch airing in the next few weeks.