I present you Markal (https://markal.app), a privacy-focused open source tool for creating interactive calendars that you can share and export to PNG/PDF. It features real-time collaboration via WebRTC, responsive UI to use it on any device, customizable legend and date ranges and stores your data locally (nothing is saved on a server).
It's made with Lit, Astro, Typescript, Deno/Express and currently is localized to English and Spanish via XLIFF, meaning it can transparently support even more languages.
You're more than welcome to contribute to this tool, self-host it, fork it, use it within your own app :D