I can confirm that whilst it is possible to get your actions within a Workflow to only execute on specific days, it is not possible to have the Workflow itself run on specific days only.
If you'd like, you can head over to our Ideas Forum to post this as a new idea for review.
If I login to Hubspot and veiw the workflow after I have created it via the API, I can goto settings and set it for just Friday from 17:00-18:00. I have attached this screenshot.
If I understand your response, this can not be configured via the API?
That is correct, we can include parameters such as onlyExecOnBizDays but this is a boolean and does not allow you to drilldown further into specific execution timestamps like the UI.
It's worth noting that the new v3 endpoints for Workflows is still in-development and so we may see extended functionality made available in future!