Hi Ferdinand,
Unfortunately, the date picker is built-in widget of browser, the only customization we can add to is setting its earliest and latest date to accept.
(It is "date" type input element.)
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/date
Therefore, the only option for us is to inject a custom date picker to the date field.
However, due to Dynamics will always set date type field to date type input element, we have to create a single line text field to support custom date picker,
then triggering a plugin/flow on update of Contact and on change of the text field to sync string date to real date field.
Here are steps:
1. Add required libraries to source code of marketing page.(In its head tag.)
* jquery.min.js
* jquery-ui.js
* jquery-ui.css
landing page

2. After form render, inject date picker of jQueryUI to the custom single line text field.
(We can get its id in HTML editor of marketing form, or with browser inspector.)
Because date picker of jQueryUI is supported to disable specific dates.

3. Now in marketing form, Sunday has been disabled in calendar.

Regards,
Clofly