Free and Open Source Software

Give us an example of what you want as the end result - what devices you have, are you sharing calendars with someone else, etc.

My best answer is to run a calendar server on some machine and let your calendar sync to that whenever the devices are online on the same wifi simultaneously. (E.G. Run Owncloud in a docker on your laptop).

Alternatively you could run Tailscale on the devices which would provide a secure mesh network, eliminating the need to be on the same wifi - so long as they're online they can sync via Tailscale.

Tailscale even has a feature (Funnel) that will route specific internet traffic into your Tailscale net - this would eliminate the need to have Tailscale on every device. You could host a calendar on a laptop (say Nextcloud in Docker with Tailscale), enable Funnel only for the calendar port, and apply security in Tailscale so only you have access.