There are cases where an entity's weekend arrangements might not simply be days of the week i.e. SAT, SUN or FRI, SAT but something else, while not in use nowadays was used historically i.e.

• Taiwan's "Every Sunday, 2nd and 4th Saturday of the month" Arrangements in 1998-2000: [1] [2] [3] - implemented in Update Taiwan holidays: add support for 1998-2000 special weekend pattern #2948
  

• Thailand's "Every Sunday and Every Buddhist Sabbath Day" as used between 1956-1957 (IMO allowing manual addition of weekends might be easier in this particular implementation) [1] [2] - implemented in Update Thailand holidays: add support for historical weekend patterns #2951
  

These features are nice to have, but not urgently needed as none of the 160+ entities currently adopt such weekend arrangements.