SurveyJS: How to get weekday of an date?


I am new here and hope am right here. If not, please forgive me. :slightly_smiling_face:

I am testing around in SurveyJS (Survey Creator - Create survey, quiz, forms without registration) and I like to create a survey with some date infos:

At first I added a single input element and set it to date.
Then I added a read-only expression and there I need an expression which gives me the weekday of that previously set day.

How do I do that?
Sorry, my JavaScript knowledges are still very limited.

Kind regards,