How and where is that done?
I can't find that functionality in proton calendar.
Only "create" and " import from google", but not something like "add from URL" (Android App).

How/who can mark it as done?

It is now possible to subscribe to ICS urls.

Yes. This would make it possible to access my Proton calendar from android's native calendar app (as well as others).

The existing calendar sharing option in Proton ought to be renamed to "export," since that's what it really does - export an ics file, which is pretty much useless as a sharing tool.

Would be more useful if you ICS link with password is included. e.g., Enterprise Gmail may not have a plain private calendar link to subscribe.

I'm quite unable to add more to what others said before me, besides "please please do this !!"

Absolutely, I third this! I am loving Proton Calendar and would like to move over completely from Gcal, but this is one feature that I am lacking. I'd like to be able to see my work and school calendars and consolidate all within Proton Calendar.

Importing calendars is a start, but subscribing is what most people need. I am trying to migrate to Proton for mail/calendar from Google but I won't be moving my wife and our current shared calendar setup isn't possible without me being able to subscribe to her calendar.

Same boat. I got about 10 calendars that are web-based.

Absolutely yes! This is essential to enable adoption as a business tool. Otherwise you need to export your encrypted events to another insecure calendar in order to see your schedule in one place. Much better to import the insecure / externally scheduled events to your private and "secure" schedule.

Seconded. One of the reasons I can't yet leave GCal is that I can subscribe to ics calendars such as for sports teams, etc. Specifically I would love to be able to subscribe to my Todoist ics so that my "to do" items were automatically on the calendar. Or better yet, contact the Todoist people and find out how to all users to tag their To-do list items with "Protoncal" like we can with Gcal and have two way integration so if I change the item on my Google calendar, it auto updates the items in Todoist.

That would be very helpful as I would be able to use ProtonCalendar as my primary app then. Currently I'm disabling encryption with full external link sharing.

I use a number of other services that provide ICS feeds for calendar events (Remember the Milk, Basecamp, etc.). It would be really handy to be able to add those URLs as calendars to my calendar so that I can view all upcoming events and tasks in one place.

I love this feature request.
It's currently labelled "ProtonMail", however it should be labelled "Proton Calendar"

This is really the same proposal as here: https://protonmail.uservoice.com/forums/284483-protonmail/suggestions/41975749-calendar-subscriptions#comments

I keep trying to get family members to use proton calendar but they say it is too hard because right now their work uses google calendar and they cannot see their work stuff here. If you allowed viewing of other calendar feeds within the proton interface, it would greatly increase adoption by users who have reservations about switching their current calendar.

This would be an amazing feature, and would greatly help users like myself while transitioning from other calendars like Google Calendar.

Specifically for me it would be great to have support for adding webcal feeds!

Would really like to add a calendar Feed option. For example, my children's school calendar has a Feed Link I can copy, but I can't download the calendar .ics file.

It's a MUST HAVE ...

Of course I understand that it wouldn't be encrypted, but I need to see multiple calendars at once: My personal one(s) (which I want encrypted), shared ones (with other ProtonMail users, encrypted), and also external calendars, like my company's google calendars (possibly using the ical secret link, or some other mechanism).