Queen Elizabeth II's birthday is a public holiday in the United Kingdom and many Commonwealth nations. Although the Queen's actual birthdate is Wednesday, 21 April 1926, the date on which her birthday is officially celebrated in the UK is the third Saturday in the month of June. In Canada they have Victoria Day on the Monday preceding 25 May instead, based on Queen Victoria's birthdate of 24 May.
Other dates for the Queen's birthday are the first Monday in October in Western Australia, the second Monday in June for the rest of Australia, whereas New Zealand observes it on the first Monday of June. That's a lot of birthday parties. The Queen's Birthday makes a great excuse for a long weekend--it's no coincidence that the smart people down under have moved it to a Monday.
Other countries with a Queen's Birthday are Denmark (16 April) and the Netherlands (30 April).
Oh yeah... on this day, don't forget to wish chevette a happy birthday. We knew it all along that you were royalty.