The Queen’s Birthday is celebrated on Monday 8 June in all states and territories except Western Australia. There, the Queen’s Birthday 2015 will be celebrate on Monday 28 September.
Actually, Queen Elizabeth II was born on 21 April 1926 but her birthday is celebrated on a separate date. She has been Queen since 1952 and is currently the second-longest reigning British monarch, the longest being Queen Victoria who reigned for 63 years.
In Britain, the monarch’s birthday has been celebrated since 1748 and, in Australia, every year since the First Fleet arrived in 1788. The birthday was always celebrated on the actual date of the monarch’s birthday until 1936, the year of the death of King George V, Elizabeth’s grandfather. His birthday was 3 June and the date has since stayed around early June (well, except in Western Australia where it is celebrated in September and in Queensland in 2016 when it will be moved to October).
I've been waiting to be awarded an OBE in the Queen's Birthday Honour's List ever since I became "naturalised" on 9 December 1971 (I must've been somewhat artificial until then ☺). I can't wait any longer and shall arrange for my own OBE (Out-of-Body Experience) on Monday by indulging in a few-too-many drinks raised to Her Majesty's good health.