The Queen’s Birthday is an Australian public holiday but the date varies across states and territories. Australia celebrates this holiday because it is a constitutional monarchy, with the English monarch as head of state. NSW this year celebrates on Monday, 13 June 2016.
On this day, the Queen’s Birthday honors list is released. This contains the names of the new members of the Order of Australia and other honors issued by the Australian government or the Queen. These are generally for exceptional service to Australians, Australia or humanity as a whole.
Companies choose to celebrate great achievers which may be all of their staff by catering for them as a treat and thus participating in the celebrations of other Australian achievers too.