Try [attr.disabled]="freitag? true : null" or [attr.readonly]="freitag" instead.
You are able to use attributes like [class.btn-lg]="someValue" in a similar way.
Your textbox works because of this:
The disabled attribute is another peculiar example. A button’s
disabled property is false by default so the button is enabled. When
you add the disabled attribute, its presence alone initializes the
button’s disabled property to true so the button is disabled.Adding and removing the disabled attribute disables and enables the
button. The value of the attribute is irrelevant, which is why you
cannot enable a button by writing<button disabled="false">Still Disabled</button>.
from https://angular.io/guide/template-syntax