Skip months within specific months range within given interval

Try this out for size.  The edate() function is useful for adding straight months.

a!localVariables(
  local!startDate: date(2021,06,02),
  local!endDate: date(2021,12,20),
  local!monthIntervals: 2,
  local!monthsBetween: month(local!endDate)-month(local!startDate)+12*(year(local!endDate)-year(local!startDate)),
  {
    local!startDate,
    reject(
      rule!APN_isEmpty,
      a!forEach(
        items: 1+enumerate(local!monthsBetween),
        expression: if(
          mod(fv!index,local!monthIntervals)=0,
          edate(local!startDate,fv!item),
          null
        )
      )
    )
  }
)