Nothing is more frustrating than needing a CalendarView control in your iOS app and realizing that you have to build one your self. It can take quite some time to build and even longer to find "those bugs".
While many OpenSource calendars exist on Github, this one is different. This one is an actual configurable library and not just a "drop in" component. It is 100% customizable and is currently the best one available on Github.
You can find the project here -> https://github.com/patchthecode/JTAppleCalendar
It is totally documented and full of tutorials. Its also easy to get your code up and running in less than 5 mins.
It can be installed using CocoaPods (something most iOS developers already know about).
The project is new. Leave a star github rating if it has saved you months of coding. Cheers