Best open-source alternatives to Google Calendar

Nextcloud Server is a self-hosted collaboration and file platform for individuals, teams, and organizations that want control over their data. It provides a central place to store files and also supports contacts, calendars, communication features, and sharing across devices. The project is designed for users who want cloud-like functionality on infrastructure they control, whether at home or in a company environment. It is extensible through a large app ecosystem, includes security features such as encryption and two-factor authentication, and can be deployed by self-installation, through hosting providers, or via appliances and preinstalled devices.

Radicale is a lightweight CalDAV and CardDAV server designed to store and share calendars, contacts, and related data over standard protocols. It is aimed at users and administrators who want a simple self-hosted groupware backend that works with many CalDAV and CardDAV clients without requiring a complicated setup. The project emphasizes ease of deployment, file-based storage, and extensibility. It supports calendars, to-do lists, journal entries, and business cards, and it can restrict access with authentication and secure connections with TLS. Because all data is kept on the filesystem in a simple folder structure, it is suitable for small deployments and for users who prefer straightforward backup and maintenance workflows.

Baïkal is a lightweight server project for running CalDAV and CardDAV services. It is intended for people who want to host their own calendar and contact synchronization backend, such as for use with clients like Thunderbird, Android apps, DavX5, and iOS devices. The README mainly points users to the project website for installation, upgrade, and troubleshooting guidance, and it also references community-written step-by-step tutorials in German and French. The repository itself is described as the source repository, and the project is developed by volunteers. The README does not describe the internal architecture or configuration details beyond its purpose as a calendar and address book server.

SOGo is presented as a collaborative free and open source groupware project. The README focuses on how contributors can help improve the software through documentation, translations, feature ideas, discussion, and code patches, rather than describing end-user workflows in detail. It also explains how to obtain the source code for SOGo and its related components, including SOPE and the Thunderbird connector extension, and points readers to compilation instructions in the project FAQ. The document further highlights the software’s internationalization support by listing a broad set of official translations and maintainers, suggesting a mature, community-driven project with a strong emphasis on multilingual use and contribution.

Davis is a self-hosted DAV server built on sabre/dav and Symfony 7, with Bootstrap 5 providing the web interface. It is aimed at users who need a modern CalDAV, CardDAV, and WebDAV backend along with an administrative dashboard for managing users, calendars, and address books. The project emphasizes straightforward deployment and configuration, offering container-based deployment, a Docker Compose sample, and a NixOS package/module. It supports Basic authentication plus external IMAP and LDAP providers, and includes features such as calendar sharing, mail notifications, public calendars, and an automatically maintained birthday calendar. The README also highlights that the interface is responsive, translated, and available in light and dark modes.

Xandikos is a lightweight server for CalDAV and CardDAV, aimed at users who want a simple self-hosted DAV backend with data stored in Git. It implements a broad set of WebDAV-related standards and is designed to work with common calendar and address book clients such as Thunderbird, DAVx5, Evolution, and Vdirsyncer. The project is intended for deployment as a small standalone service or behind a reverse HTTP proxy such as nginx or Apache. It supports direct HTTP listening, Unix domain sockets, and container-based deployment, and it can be initialized with default calendar and address book collections for immediate use. The README also notes that it has no multi-user support, making it more suitable for single-user or small personal setups than for shared multi-tenant environments.

EGroupware is a web-based groupware system intended for teams and organizations that need shared productivity tools. The README focuses on how to install and maintain the platform rather than describing end-user features in detail, but it makes clear that the project is designed to run as a multi-container application and that a package-manager-based deployment is the preferred approach for most users. It supports multiple deployment paths, including Linux packages, Docker for non-Linux environments, and a source-based development setup. The development workflow mentions optional app repositories, installer scripts, and update channels, which suggests a modular system with separate components and maintenance routines. The project is aimed at administrators and developers who need to deploy, extend, and keep a self-hosted groupware installation up to date.

Server for calendar sharing (CalDAV) that uses a PostgreSQL database as a data store.