How can I choose on which day an action may or may not be performed?

When scheduling an action in your automation, you have the option to perform an action on specific days of the week. Click on the checkbox Ship on specific days of the week:

Send_on_specificdays.png

Then the days of the week appear. Click on the days to schedule the specific days:

Selecting_days.png

The days are also visible in the flow at the promotion:

Example_specific_day.png