JTCalendar - JTCalendar is an iOS open source calendar control library. If you need your users to pick a date within your app, this library allows you to customize the look and animations of the calendar view. There is no built-in library for calendar views in iOS, so you need an outside source to implement. JTCalendar provides a simple and powerful way to make your calendars for how your app needs them.
PDTSimpleCalendar - PDTSimpleCalendar is another iOS open source date display and picker for iOS. What makes it different from JTCalendar is in the name - it’s simple. This library has all the customization options like JTCalendar, but requires less code to implement customizations. Picking between them depends on how much time you have and how deep you want to dive into the library code.
CalendarView - CalendarView is Android’s built-in widget for displaying and picking dates within an Android app. Since it’s built-in, it’s easy to access and requires no extra downloading. It’s theme is a standard Android calendar, but you can still customize the colors.
TimesSquare for Android - TimeSquare is an open-source calendar widget for both iOS and Android. If you want an alternative to Android’s built-in calendar view, then TimeSquare is a great pick that allows you to customize the style of the calendar unlike Android’s built-in CalendarView.