Etar - OpenSource Calendar is a free, open-source calendar application available on the Android platform. It offers a variety of features like month view, week view, day view, and agenda view. The app uses Android calendar sync and works with Google Calendar, Exchange, and more, making it easy to integrate with other calendars. The material design of the app provides a sleek and modern look, and it offers both dark and light themes for customization.

One of the most significant advantages of Etar is that it offers an offline calendar functionality that allows users to access their calendars without an internet connection. Additionally, the app is ad-free, making it an excellent option for those who want to avoid intrusive ads. Etar also supports sharing calendars via ics and allows users to import/export from SD card.

However, the app has some reported bugs, and feedback can be provided through their GitHub page. Overall, Etar - OpenSource Calendar is a reliable and robust calendar application that offers a variety of features and customization options, making it a worthy alternative to other calendar apps in the market.