The practical solution ...
Be aware that results are the integers one through seven.
(Not zero through six.)
let trivialDayStringsORDINAL = ["", "SUN","MON","TUE","WED","THU","FRI","SAT"]
// note that zero is not used
and then ...
let dow = Calendar.current.component(.weekday, from: someDate)
print( trivialDayStringsORDINAL[dow] )