How to create a recurring event on the first and third Saturday of each month in Apple Calendar?

I am sure I have done this previously but the options available now don't match what I am expecting. Currently I am trying to create a recurring event that happens the first and third Saturday of the month. When I go to Repeat > Custom > I have options for Daily, Weekly, Monthly or Yearly. I select month and then have option for each or On the... Each recurs on the selected date i.e 6th and 20th but that does not translate to the same day of the week each month - first and third Saturday. Selecting On the... allows me to select the day of the week but only one occurrence, either; first, second, third, fourth, fifth, next to last or last. I don't seem to be able to make multiple selection across those options even on my Mac holding the Command key - on iOS 18.6 and MacOS 15.5

[Re-Titled by Moderator]

Original Title: Creating recurring events on a custom schedule - first and third or second and fourth 'day' of the month