Calendar Overlay for KOReader

Calendar Overlay is a KOReader patch that shows upcoming events from an ICS calendar on your Kindle sleep screen.

Download

Displays upcoming calendar events on sleep screen
Multiple display styles: Minimal, Today's Schedule, Tomorrow's Schedule, Full Day
Auto-refresh every 30 minutes
Works with any ICS calendar feed (Google Calendar, Proton Calendar, iCloud)
Configurable through KOReader menu
Smart caching prevents excessive network requests

Get an ICS link from your calendar provider (How to get an ICS URL)
For Proton Calendar users: Share calendar via link
Download 2-zzz-calendar-overlay.lua
Open the file in a text editor
Paste your ICS link into the PUT_YOUR_ICS_URL_HERE spot (line 13)
Plug in your Kindle to your computer
Move the file to /mnt/us/koreader/patches/ (create the patches folder if it doesn't exist)
Safely eject your Kindle
Open KOReader
Go to KOReader and swipe down → Tools → More tools → Calendar Overlay to open menu and enable patch.

Basically all calendars can be exported as an ICS. Personally tested with Proton Calendar but all others should work.
Obviously, you have to have WiFi on to make this work