Open Source Calendar
A calendar only earns trust once it syncs everywhere without anyone reading it, and that's the catch: free calendars sit on servers that can mine your meetings, your contacts, and your whereabouts as easily as they sync them. The open source apps and CalDAV servers here speak the same open standards every device already understands, so you can host the sync yourself and keep your schedule - arguably the most revealing log of your life - on infrastructure that isn't quietly studying it.
Leantime
Open source project management for non-project managers, built with ADHD, dyslexia, and autism in mind
Radicale
CalDAV and CardDAV server for calendars, to-do lists, contacts, and file-system storage
Baïkal
Self-hostable CalDAV and CardDAV server for syncing calendars and contacts with DAV clients
Etar Calendar
Open source material-designed calendar for Android with offline events and multiple views
SOGo
Open source groupware with webmail, calendars, address books, and standards-based client access
WeekToDo
Minimalist weekly planner and to do list app with local storage and reminders
Fossify Calendar
Private, ad-free calendar app with customizable widgets, syncing, and flexible event planning