What constitutes the first weekend of a month is different in different countries. FFS what actuially constitutes a "weekend" can be different too. Some countries have Friday/Saturday weekends (Bahrain for example) and some have Thursday/Friday (Iran). Some have a six day working week with only one weekend day (India/Thailand)
I have always regarded the first weekend day (Saturday or Sunday) to be in the first weekend of any month. That is a cultural thing, and just because we do it differently here does not make us wrong. I find the Anglo-centric mocking of other's traditions exhibited in this thread to be distasteful.
In England you have "Bank Holiday Mondays", some of them without any apparent reason, e.g. the "Early May", "Spring" and "Summer" bank holidays. Its a cultural tradition which you share with the USA as it happens, but not with NZ or Australia. Yes, we have Mondays when the banks are closed too, but they always observe, celebrate or commemorate something, e.g, in NZ we have Easter Monday, Queen's Birthday, Labour Day, Christmas Day. Boxing Day, New Year's Day, and sometimes ANZAC Day (April 25) and Waitangi Day (February 6), if they happen to fall on a Monday. Individual Provincial anniversaries are also observed on Mondays.
Once again the iRB has shown its inability to write Regulations/Laws unambiguously. Simply using "first full weekend" is not enough for the reasons I mentioned earlier. IMO, they should simply drop the reference to "weekend" and specify the actual days, e.g. something like "The June window shall run from, and including, the first Saturday to, and including, the Sunday after the third Saturday". This will work unambiguously for every month of every year.