Simple Calendar (FOSS)

Simple Calendar is a popular open-source calendar app for Android that focuses on simplicity and privacy. It contains no ads, no unnecessary permissions, and works entirely offline if desired. It is the perfect choice for Android users who want a clean, lightweight calendar that doesn't track their data or drain their battery. While it lacks the complex cloud-syncing features of Google Calendar, it is highly reliable for local event management and is a favorite among the privacy-conscious Android community.

It is a testament to the power of simple, well-coded software.