The day is celebrated in all 54 commonwealth countries or states every year at different times to mark the birthday of the Queen (for a list of commonwealth countries click here)
It is not her official birthday however. Queen Elizabeth’s real birthday is April 21.
If Prince Charles or Prince William become the next sovereign, the name of the day will change to 'King’s Birthday'.
In New Zealand the day always falls on the first weekend of June.
In 2009 a group of business people petitioned the government to change the name of the weekend to Hilliary Weekend (after Sir Edmund Hillary) but this was unsuccessful.
Queen Elizabeth has been Queen for 58 years!
Click here for to visit the official website for the royal family. It's full of photos, videos and interesting facts about the Queen. It also has her postal address so you can actually write a letter to the Queen!