Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...
Lightning Calendar - Organize your life — it’s about time!
Etar - Material design open source calendar for Android.
Calendar Lock PEA - Calendar Lock PEA is an encrypted desktop calendar with a day, week and month view. The shown calendars are never stored unencrypted on your disk, but exist only in the RAM. Platform-independent, Open Source, no installation or registration required.
Google Calendar - Spend less time managing your day & more time enjoying it
UKs Kalender - This program helps you organizing your dates and tasks and reminds you of upcoming events.