class=hidden is used in lots of places in the frontend. So the proposed solution is just a band aid for one possible issue and not a solution for the non-core template not having the required class

The calendar should be encapsulated and work everywhere.
If a template does not have the hidden class it will not work.
This is a simple fix to ensure functionality everywhere.

This is a simple fix to ensure functionality everywhere.

so submit a pull request - dont expect others to do it for you

@angieradtke It needs to change it in the "build/media_source" here https://github.com/joomla/joomla-cms/blob/4.0-dev/build/media_source/system/css/fields/calendar.css , not in the "media" folder.

@richard67 You certainly know better than me where to change .
I'm tinkering a page here from the scratch with all possible functionalities and note possible errors.

@richard67 You certainly know better than me where to change .

@angieradtke Well now you know it too ? Do you want to make a PR or shall someone else do it?

so submit a pull request - dont expect others to do it for you

I think I can help more by pointing out the things I find while experimenting.
@richard67
I would do it, but for you it is easy. It is not my daily buisness, so I forgot every time how it works.
For me it takes hours. Checking out the stuff to may local computer, etc...
With svn it was easy creating a patch done, but ....

The code has a class hidden because at that time IE8 and IE9 were still supported. These days an attribute hidden will do the work without any lines of CSS.

My 2c

I'd prefer someone else than me to make a PR.

@dgrammatiko @richard67
Yes, as long as we don't have an attribute hidden and we use the class we also need appropriate CSS.

@Fedik is already patching the calendar right now for the strings, ask him to change the toggling of the class to a toggle of a hidden attribute...

Eg change

2 lines of code can be done in the Github by editing the file...

this may be related #34791, just for reference

Please test #34871