People don’t seem to care about this category since Google took over the world with its calendar. I’ve never liked most calendars out there for more than just the privacy concerns. The UI is often very uninspired and doesn’t encourage you to use it.
Fortunately, there are better options to get more organized while maintaining your privacy.
Desktop
Lightning Calendar
Lightning is a fully-featured calendar add-on for the Thunderbird desktop mail client. Like Thunderbird (and thereby Betterbird, it supports Windows, MacOS and Linux.
BORG
If you don’t want an email client, BORG is a cross-platform FOSS calendar. It supports CalDAV sync, advanced task management, and even address books.
Rainlendar
Raindrop is not open-source.
Rainlendar is not open-source. With that out of the way, it’s good-looking, customizable and supports CalDAV. It’s available for all desktop platforms. While they do offer a limited free version, more advanced features will set you back a few dollars.
I’m only including Rainlendar due to the lack of desktop options. But you should try the FOSS options here before settling for this one.
Linux
Linux users are spoiled with calendar options. Here’s two good-looking ones that support CalDAV: Calindori and Merkuro. Both support Plasma Mobile as well.
Android
MyBrain
If you don’t care about sync functionality, MyBrain is an all-in-one productivity app. It combines calendar, notes, tasks, and bookmarks in a good-looking Material Design app. Sadly, it’s only available for Android and doesn’t support any kind of synchronization.
Etar
Etar is an open-source fork of Android’s built-in calendar. It uses Android’s built-in Calendar Sync, so CalDAV is available using DAVx5. If you want a drop-in replacement for Google’s calendar, you’ll like this one.
Simple Calendar
Don’t be fooled by the name. Simple Calendar is actually quite feature-rich and customizable, while remaining snappy and lightweight. Like the rest of Simple Mobile Tools’ apps, it’s open-source, can work completely offline if you want it to or sync it to a server of your choice.
The free version is no longer being updated and won’t receive any new features. The developer has decided to only continue improving the Pro version, which is only $1.19 and already has several new features that the free version lacks.
Update 2023:
SimpleMobileTools have been acquired by ZippoApps. They are now adware and likely spyware. Fortunately, Simple Calendar is revived by a fork called Calendar.
aCalendar
aCalendar is not open-source.
aCalendar is an advanced calendar app for Android. It integrates tasks and subtasks into your calendar, both of which you can sync to your server. As long as it supports vTODO).
The downside is that it’s not open-source, and some features are in-app purchases.
Calendar
I know. It’s like these names keep getting worse. Do these developers not care about discoverability? Anyway. Calendar takes some design cues from the iOS calendar, but with a Material Design twist. It’s a very good looking calendar. Of course it supports CalDAV, has beautiful widgets, and is highly-customizable. So much creativity until they got to the name.
iOS
I don’t use iOS devices, so I can’t provide recommendations here. Here’s a curated list of FOSS Calendar apps for iOS. Some of them are on the App Store while others might need compiling. That said, the native iOS calendar app supports CalDAV apparently.
Self-hosted
Once upon a time, there was a bunch of open-source web-based calendar projects. Most seem to been abandoned many years ago, likely in favor of Google’s offering.
Manage My Damn Life
MMDL stands out as the last of a dying breed. It has a beautiful interface with task integration and CalDAV support. For my use case, it suits my needs perfectly. My only complaint is the amount of storage space it takes up at 1.4GB. For context, my second largest website takes up a third of that. Most of my other websites get by with about a tenth of that.
Cal.com
Cal.com looks even better than MMDL, and has even more features. It includes built-in scheduling of appointments and even receiving payments. Problem is, it takes up even more space than MMDL. I’m using 3 different services to get the functionality it offers and all 3 take up far less space than it does alone. I understand this is a personal gripe, but I like efficiency. It’s very pretty though.
Nextcloud
Nextcloud has a first-party calendar app. It allows you to create events, invite people, and even sync to any CalDAV client. It can also auto-populate birthdays by default so you always know when one is coming up.
You can learn more about Nextcloud in the Google Drive alternatives article. After all, that’s its main function.
Sogo
Sogo is not exclusively a calendar either. It’s a complete groupware suite. The only reason I’m mentioning it here is due to the severe lack of options in this category. I also happen to absolutely love the design of their calendar. Read more about it in the GSuite alternatives article.