Assigning levels in random order for calender dates

To create sampling schedule for an experiment, I want to assign 4 different individuals E.g. x<- ("a","b","c","d") in a random order to every Monday in a year (e.g. 1/1/18 to 31/12/19). I already have dates ready as variable Y and corresponding weekdays as Variable Z. Also, I want to assign other 4 individuals to Tuesdays in a similar way. What is the best way to do this? is it simple to use 'for' loop? Thanks.