"Queen's Birthday" is an official holiday celebrating the birthday of the monarch of the United Kingdom and the Commonwealth of Nations, which usually does not coincide with the ruler's actual birthday. The date of the holiday in different countries is set for different dates and only symbolizes the birthday of the monarch, who is currently Elizabeth II.
There are two official Queen's birthdays in Australia. Most Australians celebrate it on the second Monday in June. But the inhabitants of the state of Western Australia prefer to celebrate it every year on the Monday closest to September 30th.
In fact, Queen Elizabeth Alexandra Mary of Windsor was born on April 21, 1926. She was far from the first British monarch to postpone the celebration of her birthday to a more suitable - sunny and warm (which is very important for the UK) - time. Even Edward VII at the beginning of the 20th century decided to celebrate his birthday in summer instead of November. The Australians chose June simply because the Queen's real birthday is close to another big holiday in the country - Australian and New Zealand Army Corps Day (ANZAC Day - Australian and New Zealand Army Corps), which is celebrated on April 25th. Two official holidays in a week? For Australia, that's a lot.