The Queen's Birthday is a public holiday that celebrates the monarch of the Commonwealth, currently Queen Elizabeth II. Different countries celebrate the public holiday at a different time (that doesn't coincide with the Queen's actual day of birth) because it is traditionally celebrated in May or June when good weather can be expected in the northern hemisphere.
In Australia (with the exception of Western Australia and Queensland) the Queen's Birthday is celebrated on the second Monday in June. In 2014, this public holiday will take place on June 9th. Since the holiday always takes place on a Monday, the Queen's Birthday provides a long weekend every year and a good excuse to plan a getaway.
Check out these great ideas for each state: