Apple delivered iOS 9 to the developer world at WWDC in June 2015, before launching it to the public in September. The iOS 9 SDK introduced some big changes to the user, such as iPad multitasking and search and Siri enhancements. The majority of the new features were focused on optimization and performance enhancements. A large number of new APIs were introduced, and iOS9 Day-by-Day is a review of the most important of these. The motivation behind this is that busy developers don't have time to trawl the WWDC videos and Apple documentation. Instead, they'd like to get a high-level summary of the new possibilities, alongside some working sample code. This is exactly what iOS9 Day-by-Day provides.