You will have to store it in an additional attribute and save it as a string value. You can extract it from the date by using FormatDateTime($Date, 'EEEE') Please note that the date will be written down based on the locale of the user that executes the procedure to determine the day of the week. Alternatively you can make this a calculated value by using a virtual attribute.
I would suggest you to download in the appstore 'NonWorkingDays' module. This is a great module with flows that retrieve days/ (non)working days, convert a date to a date name e.t.c. You can use functionalitys from that module to solve your problem!